Hot-rolled steel sheet, cold-rolled steel sheet, galvanized steel sheet, and methods of manufacturing the same

ABSTRACT

A hot-rolled steel sheet has an average value of the X-ray random intensity ratio of a {100}&lt;011&gt; to {223}&lt;110&gt; orientation group at least in a sheet thickness central portion that is in a sheet thickness range of ⅝ to ⅜ from a steel sheet surface of 1.0 to 6.0, an X-ray random intensity ratio of a {332}&lt;113&gt; crystal orientation of 1.0 to 5.0, rC which is an r value in a direction perpendicular to a rolling direction of 0.70 to 1.10, and r30 which is an r value in a direction that forms an angle of 30° with respect to the rolling direction of 0.70 to 1.10.

This is a Divisional of U.S. application Ser. No. 13/811,902 filed Mar.7, 2013, which is the National Stage Entry of PCT InternationalApplication No. PCT/JP2011/067070 filed on Jul. 27, 2011, which claimspriority under 35 U.S.C. §119(a) to Japanese Patent Application Nos.2010-169230, 2010-169627, 2010-169670 filed in Japan on Jul. 28, 2010,Japanese Patent Application No. 2010-204671 filed in Japan on Sep. 13,2010, Japanese Patent Application Nos. 2011-048236, 2011-048246,2011-048253 and 2011-048272 filed in Japan on Mar. 4, 2011. Each of theabove applications are hereby expressly incorporated by reference, inits entirety, into the present application.

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet, a cold-rolledsteel sheet, and a galvanized steel sheet which are excellent in termsof local deformability, such as bending, stretch flange, or a burringworking, have a small orientation dependency of formability, and areused mainly for automobile components and the like, and methods ofmanufacturing the same. The hot-rolled steel sheet includes a hot-rolledstrip that serves as a starting sheet for the cold-rolled steel sheet,the galvanized steel sheet, or the like.

BACKGROUND ART

An attempt is being made to reduce the weight of an automobile framethrough use of a high-strength steel sheet in order to suppress theamount of carbon dioxide exhausted from an automobile. In addition, ahigh-strength steel sheet as well as a soft steel sheet has beenfrequently used for automobile frames from the viewpoint of securing thesafety of passengers. However, in order to further reduce the weight ofan automobile frame in the future, it is necessary to increase the levelof operational strength of a high-strength steel sheet compared to therelated art.

However, in general, an increase in the strength of a steel sheetresults in a decrease in the formability. For example, Non PatentDocument 1 discloses that an increase in strength degrades uniformelongation which is important for drawing or stretch forming.

Therefore, in order to use a high-strength steel sheet for underbodycomponents of an automobile frame, components that contribute toabsorption of impact energy, and the like, it becomes important toimprove local deformability, such as local ductility that contributes toformability, such as burring workability or bending workability.

In contrast to the above, Non Patent Document 2 discloses a method inwhich uniform elongation is improved by complexing the metallicstructure of a steel sheet even when the strength is maintained at thesame level.

In addition, Non Patent Document 3 discloses a metallic structurecontrol method in which local deformability represented by bendingproperties, hole expanding workability, or burring workability isimproved through inclusion control, single structure formation, and,furthermore, a decrease in the hardness difference between structures.The above method is to improve hole expanding properties by forming asingle structure through structure control, and, in order to form asingle structure, a thermal treatment from an austenite single phaseserves as the basis of the manufacturing method as described in NonPatent Document 4.

In addition, Non Patent Document 4 discloses a technique in whichmetallic structure is controlled through the control of cooling afterhot rolling, and precipitates and deformed structures are controlled soas to obtain ferrite and bainite at an appropriate proportion, therebysatisfying both an increase in the strength and securement of ductility.

However, all of the above techniques are a method of improving localdeformability through structure control, which is significantlyinfluenced by base structure formation.

Meanwhile, even for improvement of material quality through an increasein the rolling reduction in a continuous hot rolling process, relatedart exists, which is a so-called grain refinement technique. Forexample, Non Patent Document 5 describes a technique in which largereduction is carried out at an extremely low temperature range in anaustenite range, and non-recrystallized austenite is transformed intoferrite so that the crystal grains of ferrite which is the main phase ofthe product are refined, and the strength or toughness increases due tothe grain refinement. However, Non Patent Document 5 pays no attentionto improvement of local deformability which is the object of the presentinvention.

CITATION LIST Non Patent Documents

-   [Non Patent Document 1] “Nippon Steel Corporation Technical Report,”    by Kishida (1999) No. 371, p. 13-   [Non Patent Document 2] “Trans. ISIJ,” by O. Matsumura et al. (1987)    Vol. 27, P. 570-   [Non Patent Document 3] “Steel-manufacturing studies,” by Kato et    al. (1984) Vol. 312, p. 41-   [Non Patent Document 4] “ISIJ International,” by K. Sugimoto et    al. (2000) Vol. 40, p. 920-   [Non Patent Document 5] NFG Catalog, Nakayama Steel Works, Ltd.

SUMMARY OF INVENTION Technical Problem

As described above, structure control including inclusion control was amain solution for improving the local deformability of a high-strengthsteel sheet. However, since the solution relied on structure control, itwas necessary to control the proportion or form of structures, such asferrite and bainite, and the base metallic structure was limited.

Therefore, in the present invention, control of a texture is employedinstead of control of the base structure, and a hot-rolled steel sheet,a cold-rolled steel sheet, and a galvanized steel sheet which areexcellent in terms of the local deformability of a high-strength steelsheet, and have a small orientation dependency of formability, and amethod of manufacturing the same are provided by controlling the size orform of crystal grains and texture as well as the kinds of phases.

Solution to Problem

According to the knowledge in the related art, hole expandingproperties, bending properties, and the like were improved throughinclusion control, precipitation refinement, structure homogenization,formation of a single structure, a decrease in the hardness differencebetween structures, and the like. However, with the above techniquesalone, the main structure composition will be limited. Furthermore, in acase in which Nb, Ti, and the like which are typical elements thatsignificantly contribute to an increase in strength are added in orderto increase the strength, since there is a concern that anisotropy mayincrease extremely, it is necessary to sacrifice other forming factorsor limit the direction in which blanks are taken before forming, therebylimiting uses.

Therefore, the present inventors newly paid attention to the influenceof the texture in a steel sheet in order to improve hole expandingproperties or bending workability, and investigated and studied theeffects in detail. As a result, the inventors clarified that localdeformability is drastically improved by controlling the X-ray randomintensity ratio of the respective orientations of a specific crystalorientation group from a hot rolling process, and, furthermore,controlling the r value in a rolling direction, the r value in thedirection perpendicular to the rolling direction, and the r value in adirection that forms an angle of 30° or 60° with respect to the rollingdirection.

The present invention was constituted based on the above finding, andthe present invention employed the following measures in order to solvethe above problems and achieve the relevant object.

(1) That is, a hot-rolled steel sheet according to an aspect of thepresent invention contains, by mass %, C: 0.0001% to 0.40%, Si: 0.001%to 2.5%, Mn: 0.001% to 4.0%, P: 0.001% to 0.15%, S: 0.0005% to 0.03%,Al: 0.001% to 2.0%, N: 0.0005% to 0.01%, and O: 0.0005% to 0.01%, andfurther contains one or two or more of Ti: 0.001% to 0.20%, Nb: 0.001%to 0.20%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, B: 0.0001% to 0.0050%,Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Cu: 0.001% to 2.0%, Ni: 0.001%to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Zr: 0.0001% to 0.2%,As: 0.0001% to 0.50%, Mg: 0.0001% to 0.010%, Ca: 0.0001% to 0.010%, andREM: 0.0001% to 0.1% and balance composed of iron and inevitableimpurities, in which an average value of an X-ray random intensity ratioof a {100}<011> to {223}<110> orientation group at least in a thicknesscentral portion that is in a sheet thickness range of ⅝ to ⅜ from asteel sheet surface is 1.0 to 6.0, an X-ray random intensity ratio of a{332}<113> crystal orientation is 1.0 to 5.0, rC which is an r value ina direction perpendicular to a rolling direction is 0.70 to 1.10, andr30 which is an r value in a direction that forms an angle of 30° withrespect to the rolling direction is 0.70 to 1.10.

(2) In addition, in the aspect according to the above (1), furthermore,rL which is an r value in the rolling direction may be 0.70 to 1.10, andr60 which is an r value in a direction that forms an angle of 60° withrespect to the rolling direction may be 0.70 to 1.10.

(3) In addition, in the aspect according to the above (1) or (2),furthermore, one or two or more of bainite, martensite, pearlite, andaustenite are present in the hot-rolled steel sheet, and a proportion ofgrains having a dL/dt which is a ratio of a length in the rollingdirection dL to a length of a sheet thickness direction dt of 3.0 orless in crystal grains in the structures may be 50% to 100%.

(4) In the aspect according to the above (1) or (2), an area proportionof crystal grains having a grain diameter of more than 20 μm in a totalarea of a metallic structure in the hot-rolled steel sheet may be 0% to10%.

(5) A cold-rolled steel sheet according to an aspect of the presentinvention is a cold-rolled steel sheet obtained through cold rolling ofthe hot-rolled steel sheet according to the above (1), in which theaverage value of the X-ray random intensity ratio of a {100}<011> to{22}<110> orientation group at least in the thickness central portion is1.0 to less than 4.0, the X-ray random intensity ratio of a {332}<113>crystal orientation is 1.0 to 5.0, rC which is the r value in adirection perpendicular to the rolling direction is 0.70 to 1.10, andr30 which is the r value in a direction that forms an angle of 30° withrespect to the rolling direction is 0.70 to 1.10.

(6) In the aspect according to the above (5), rL which is an r value inthe rolling direction may be 0.70 to 1.10, and r60 which is an r valuein a direction that forms an angle of 60° with respect to the rollingdirection may be 0.70 to 1.10.

(7) In the aspect according to the above (5) or (6), furthermore, one ortwo or more of bainite, martensite, pearlite, and austenite are presentin the cold-rolled steel sheet, and a proportion of grains having adL/dt which is a ratio of a length in the rolling direction dL to alength of a sheet thickness direction dt of 3.0 or less in crystalgrains in the structures may be 50% to 100%.

(8) In the aspect according to the above (5) or (6), an area proportionof crystal grains having a grain diameter of more than 20 μm in a totalarea of a metallic structure in the cold-rolled steel sheet may be 0% to10%.

(9) A galvanized steel sheet according to an aspect of the presentinvention is a galvanized steel sheet further having a galvanizedcoating layer or a galvanealed coating layer on a surface of thecold-rolled steel sheet according to the above (5), in which the averagevalue of the X-ray random intensity ratio of a {100}<011> to {223}<110>orientation group at least in the thickness central portion is 1.0 toless than 4.0, the X-ray random intensity ratio of a {332}<113> crystalorientation is 1.0 to 5.0, rC which is the r value in a directionperpendicular to the rolling direction is 0.70 to 1.10, and r30 which isthe r value in a direction that forms an angle of 30° with respect tothe rolling direction is 0.70 to 1.10.

(10) In the aspect according to the above (9), rL which is an r value inthe rolling direction may be 0.70 to 1.10, and r60 which is an r valuein a direction that forms an angle of 60° with respect to the rollingdirection may be 0.70 to 1.10.

(11) In a method of manufacturing the hot-rolled steel sheet accordingto an aspect of the present invention, first hot rolling in which aningot or slab which contains, by mass %, C: 0.0001% to 0.40%, Si: 0.001%to 2.5%, Mn: 0.001% to 4.0%, P: 0.001% to 0.15%, S: 0.0005% to 0.03%,Al: 0.001% to 2.0%, N: 0.0005% to 0.01%, and O: 0.0005% to 0.01%, andfurther contains one or two or more of Ti: 0.001% to 0.20%, Nb: 0.001%to 0.20%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, B: 0.0001% to 0.0050%,Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Cu: 0.001% to 2.0%, Ni: 0.001%to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Zr: 0.0001% to 0.2%,As: 0.0001% to 0.50%, Mg: 0.0001% to 0.010%, Ca: 0.0001% to 0.010%, andREM: 0.0001% to 0.1% and balance composed of iron and inevitableimpurities is rolled at least once at a rolling reduction ratio of 20%or more is carried out in a temperature range of 1000° C. to 1200° C.,an austenite grain diameter is set to 200 μm or less, second hot rollingin which a total of rolling reduction ratios is 50% or more is carriedout in a temperature range of T1+30° C. to T1+200° C., third hot rollingin which a total of rolling reduction ratios is less than 30% is carriedout in a temperature range of T1° C. to T1+30° C., and hot rolling endsat an Ar3 transformation temperature or higher.

Here, T1 is a temperature determined by steel sheet components, andexpressed by the following formula 1.

T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (Formula1)

(12) In the aspect according to the above (11), in the second hotrolling in the temperature range of T1+30° C. to T1+200° C., the ingotor slab may be rolled at least once at a rolling reduction ratio of 30%or more in a pass.

(13) In the aspect according to the above (11) or (12), in the first hotrolling in a temperature range of 1000° C. to 1200° C., the ingot orslab may be rolled at least twice at a rolling reduction ratio of 20% ormore, and the austenite grain diameter may be set to 100 μm or less.

(14) In the aspect according to the above (11) or (12), in a case inwhich the pass in which the rolling reduction ratio is 30% or more inthe temperature range of T1+30° C. to T1+200° C. is defined as a largereduction pass, a waiting time t from completion of a final pass of thelarge reduction pass to initiation of cooling may employ a configurationthat satisfies the following formula 2.

t1≦t≦t1×2.5  (Formula 2)

Here, t1 is expressed by the following formula 3.

t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 3)

Here, Tf represents a temperature after the final pass, and P1represents a rolling reduction ratio in the final pass.

(15) In the aspect according to the above (14), a temperature of thesteel sheet may increase by 18° C. or less between the respective passesof the second hot rolling in the temperature range of T1+30° C. toT1+200° C.

(16) In a method of manufacturing the cold-rolled steel sheet accordingto an aspect of the present invention, after the end of the hot rollingat the Ar3 transformation temperature or higher, the hot-rolled steelsheet obtained through the method of manufacturing the hot-rolled steelsheet according to the above (11) is pickled, cold-rolled at 20% to 90%,annealed at a temperature range of 720° C. to 900° C. for a holding timeof 1 second to 300 seconds, acceleration-cooled at a cooling rate of 10°C./s to 200° C./s from 650° C. to 500° C., and held at a temperature of200° C. to 500° C.

(17) In the aspect according to the above (16), in the second hotrolling in the temperature range of T1+30° C. to T1+200° C., rolling ata rolling reduction ratio of 30% or more in a pass may be carried out atleast once.

(18) In the aspect according to the above (16) or (17), in the first hotrolling in the temperature range of 1000° C. to 1200° C., rolling at arolling reduction ratio of 20% or more may be carried out at leasttwice, and the austenite grain diameter may be set to 100 μm or less.

(19) In the aspect according to the above (16) or (17), in a case inwhich the pass in which the rolling reduction ratio is 30% or more inthe temperature range of T1+30° C. to T1+200° C. is defined as a largereduction pass, a waiting time t from completion of a final pass of thelarge reduction pass to initiation of cooling may employ a configurationthat satisfies the following formula 4.

t1≦t≦t1×2.5  (Formula 4)

Here, t1 is expressed by the following formula 5.

t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 5)

Here, Tf represents a temperature after the final pass, and P1represents a rolling reduction ratio in the final pass.

(20) In the aspect according to the above (16) or (17), a temperature ofthe steel sheet may increase by 18° C. or less between the respectivepasses of the second hot rolling in the temperature range of T1+30° C.to T1+200° C.

(21) In a method of manufacturing the galvanized steel sheet accordingto an aspect of the present invention, after the end of the hot rollingat the Ar3 transformation temperature or higher, the hot-rolled steelsheet obtained through the method of manufacturing the hot-rolled steelsheet according to the above (11) is wound in a temperature range of680° C. to room temperature, pickled, cold-rolled at 20% to 90%, heatedto a temperature range of 650° C. to 900° C., annealed for a holdingtime of 1 second to 300 seconds, cooled at a cooling rate of 0.1° C./sto 100° C./s from 720° C. to 580° C., and a galvanizing treatment iscarried out.

(22) In the aspect according to the above (21), in the second hotrolling in the temperature range of T1+30° C. to T1+200° C., rolling ata rolling reduction ratio of 30% or more in a pass may be carried out atleast once.

(23) In the aspect according to the above (21) or (22), in the first hotrolling in the temperature range of 1000° C. to 1200° C., rolling at arolling reduction ratio of 20% or more may be carried out at leasttwice, and the austenite grain diameter may be set to 100 μm or less.

(24) In the aspect according to the above (21) or (22), in a case inwhich the pass in which the rolling reduction ratio is 30% or more inthe temperature range of T1+30° C. to T1+200° C. is defined as a largereduction pass, a waiting time t from completion of a final pass of thelarge reduction pass to initiation of cooling may employ a configurationthat satisfies the following formula 6.

t1≦t≦t1×2.5  (Formula 6)

Here, t1 is expressed by the following formula 7.

t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 7)

Here, Tf represents a temperature after the final pass, and P1represents a rolling reduction ratio in the final pass.

(25) In the aspect according to the above (24), a temperature of thesteel sheet may increase by 18° C. or less between the respective passesof the second hot rolling in the temperature range of T1+30° C. toT1+200° C.

Advantageous Effects of Invention

According to the present invention, without limiting the main structurecomponents, it is possible to obtain a hot-rolled steel sheet, acold-rolled steel sheet, and a galvanized steel sheet which have a smallinfluence on anisotropy even when elements, such as Nb or Ti, are added,are excellent in terms of local deformability, and have a smallorientation dependency of formability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the relationship between the average value ofan X-ray random intensity ratio of a {100}<011> to {223}<110>orientation group and the sheet thickness/minimum bending radius of ahot-rolled steel sheet.

FIG. 2 is a view showing the relationship between the average value ofan X-ray random intensity ratio of a {332}<113> crystal orientation andthe sheet thickness/minimum bending radius of the hot-rolled steelsheet.

FIG. 3 is a view showing the relationship between rC which is an r valuein a direction perpendicular to a rolling direction and the sheetthickness/minimum bending radius of the hot-rolled steel sheet.

FIG. 4 is a view showing the relationship between r30 which is an rvalue in a direction that forms an angle of 30° with respect to therolling direction and the sheet thickness/minimum bending radius of thehot-rolled steel sheet.

FIG. 5 is a view showing the relationship between rL which is an r valuein the rolling direction and the sheet thickness/minimum bending radiusof the hot-rolled steel sheet.

FIG. 6 is a view showing the relationship between r60 which is an rvalue in a direction that forms an angle of 60° with respect to therolling direction and the sheet thickness/minimum bending radius of thehot-rolled steel sheet.

FIG. 7 is a view showing the relationship between the average value ofthe X-ray random intensity ratio of a {100}<011> to {22}<110>orientation group and the sheet thickness/minimum bending radius of acold-rolled steel sheet.

FIG. 8 is a view showing the relationship between the average value ofthe X-ray random intensity ratio of the {332}<113> crystal orientationand the sheet thickness/minimum bending radius of the cold-rolled steelsheet.

FIG. 9 is a view showing the relationship between rC which is the rvalue in the direction perpendicular to the rolling direction and thesheet thickness/minimum bending radius of the cold-rolled steel sheet.

FIG. 10 is a view showing the relationship between r30 which is the rvalue in the direction that forms an angle of 30° with respect to therolling direction and the sheet thickness/minimum bending radius of thecold-rolled steel sheet.

FIG. 11 is a view showing the relationship between rL which is the rvalue in the rolling direction and the sheet thickness/minimum bendingradius of the cold-rolled steel sheet.

FIG. 12 is a view showing the relationship between r60 which is the rvalue in the direction that forms an angle of 60° with respect to therolling direction and the sheet thickness/minimum bending radius of thecold-rolled steel sheet.

FIG. 13 is a view showing the relationship between the average value ofthe X-ray random intensity ratio of a {100}<011> to {22}<110>orientation group and the sheet thickness/minimum bending radius of agalvanized steel sheet.

FIG. 14 is a view showing the relationship between the average value ofthe X-ray random intensity ratio of the {332}<113> crystal orientationand the sheet thickness/minimum bending radius of the galvanized steelsheet.

FIG. 15 is a view showing the relationship between rC which is the rvalue in the direction perpendicular to the rolling direction and thesheet thickness/minimum bending radius of the galvanized steel sheet.

FIG. 16 is a view showing the relationship between r30 which is the rvalue in the direction that forms an angle of 30° with respect to therolling direction and the sheet thickness/minimum bending radius of thegalvanized steel sheet.

FIG. 17 is a view showing the relationship between rL which is the rvalue in the rolling direction and the sheet thickness/minimum bendingradius of the galvanized steel sheet.

FIG. 18 is a view showing the relationship between r60 which is the rvalue in the direction that forms an angle of 60° with respect to therolling direction and the sheet thickness/minimum bending radius of thegalvanized steel sheet.

FIG. 19 is a view showing the relationship between the austenite graindiameter after rough rolling and rC which is the r value in thedirection perpendicular to the rolling direction in the hot-rolled steelsheet.

FIG. 20 is a view showing the relationship between the austenite graindiameter after rough rolling and r30 which is the r value in thedirection that forms an angle of 30° with respect to the rollingdirection in the hot-rolled steel sheet.

FIG. 21 is a view showing the relationship between the number of timesof rolling at a rolling reduction ratio of 20% or more in rough rollingand the austenite grain diameter after the rough rolling.

FIG. 22 is a view showing the relationship between a total rollingreduction ratio in a temperature range of T1+30° C. to T1+200° C. andthe average value of the X-ray random intensity ratio of a {100}<011> to{223}<110> orientation group in the hot-rolled steel sheet.

FIG. 23 is a view showing the relationship between a total rollingreduction ratio in a temperature range of T1° C. to lower than T1+30° C.and the average value of the X-ray random intensity ratio of a{100}<011> to {22}<110> orientation group in the hot-rolled steel sheet.

FIG. 24 is a view showing the relationship between a total rollingreduction ratio in a temperature range of T1+30° C. to T1+200° C. andthe X-ray random intensity ratio of the {332}<113> crystal orientationin the hot-rolled steel sheet.

FIG. 25 is a view showing the relationship between a total rollingreduction ratio in a temperature range of T1° C. to lower than T1+30° C.and the X-ray random intensity ratio of the {332}<113> crystalorientation in the hot-rolled steel sheet.

FIG. 26 is a view showing the relationship among a maximum temperatureincrease amount of the steel sheet between the respective passes duringrolling in a temperature range of T1+30° C. to T1+200° C., a waitingtime from completion of a final pass of the large reduction pass toinitiation of cooling in a case in which the pass in which the rollingreduction ratio is 30% or more in the temperature range of T1+30° C. toT1+200° C. is defined as a large reduction pass, and rL which is the rvalue in the rolling direction in the hot-rolled steel sheet.

FIG. 27 is a view showing the relationship among a maximum temperatureincrease amount of the steel sheet between the respective passes duringrolling in a temperature range of T1+30° C. to T1+200° C., a waitingtime from completion of a final pass of the large reduction pass toinitiation of cooling in a case in which the pass in which the rollingreduction ratio is 30% or more in the temperature range of T1+30° C. toT1+200° C. is defined as a large reduction pass, and r60 which is the rvalue in the direction that forms an angle of 60° with respect to therolling direction in the hot-rolled steel sheet.

FIG. 28 is a view showing the relationship between the austenite graindiameter after the rough rolling and rC which is the r value in thedirection perpendicular to the rolling direction in the cold-rolledsteel sheet.

FIG. 29 is a view showing the relationship between the austenite graindiameter after the rough rolling and r30 which is the r value in thedirection that forms an angle of 30° with respect to the rollingdirection in the cold-rolled steel sheet.

FIG. 30 is a view showing the relationship between the rolling reductionratio of T1+30° C. to T1+200° C. and the average value of the X-rayrandom intensity ratio of a {100}<011> to {223}<110> orientation groupin the cold-rolled steel sheet.

FIG. 31 is a view showing the relationship between the total rollingreduction ratio in a temperature range of T1+30° C. to T1+200° C. andthe X-ray random intensity ratio of the {332}<113> crystal orientationin the cold-rolled steel sheet.

FIG. 32 is a view showing the relationship between the austenite graindiameter after the rough rolling and rC which is the r value in theperpendicular direction to the rolling direction in a galvanized steelsheet.

FIG. 33 is a view showing the relationship between the austenite graindiameter after the rough rolling and r30 which is the r value in thedirection that forms an angle of 30° with respect to the rollingdirection in the galvanized steel sheet.

FIG. 34 is a view showing the relationship between the total rollingreduction ratio in a temperature range of T1+30° C. to T1+200° C. andthe average value of the X-ray random intensity ratio of the {100}<011>to {223}<110> orientation group in the galvanized steel sheet.

FIG. 35 is a view showing the relationship between the total rollingreduction ratio in a temperature range of T1° C. to lower than T1+30° C.and the average value of the X-ray random intensity ratio of the{100}<011> to {223}<110> orientation group in the galvanized steelsheet.

FIG. 36 is a view showing the relationship between the total rollingreduction ratio in a temperature range of T1+30° C. to T1+200° C. andthe X-ray random intensity ratio of the {332}<113> crystal orientationin the galvanized steel sheet.

FIG. 37 is a view showing the relationship between the total rollingreduction ratio in a temperature range of T1° C. to lower than T1+30° C.and the X-ray random intensity ratio of the {332}<113> crystalorientation in the galvanized steel sheet.

FIG. 38 is a view showing the relationship among a maximum temperatureincrease amount of the steel sheet between the respective passes duringrolling in a temperature range of T1+30° C. to T1+200° C., the waitingtime from completion of a final pass of the large reduction pass toinitiation of cooling in a case in which the pass in which the rollingreduction ratio is 30% or more in the temperature range of T1+30° C. toT1+200° C. is defined as a large reduction pass, and rL which is the rvalue in the rolling direction in the galvanized steel sheet.

FIG. 39 is a view showing the relationship among a maximum temperatureincrease amount of the steel sheet between the respective passes duringrolling in a temperature range of T1+30° C. to T1+200° C., a waitingtime from completion of a final pass of the large reduction pass toinitiation of cooling in a case in which the pass in which the rollingreduction ratio is 30% or more in the temperature range of T1+30° C. toT1+200° C. is defined as a large reduction pass, and r60 which is the rvalue in the direction that forms an angle of 60° with respect to therolling direction in the galvanized steel sheet.

FIG. 40 is a view showing the relationship between strength and holeexpanding properties of the hot-rolled steel sheet of the embodiment anda comparative steel.

FIG. 41 is a view showing the relationship between strength and bendingproperties of the hot-rolled steel sheet of the embodiment and thecomparative steel.

FIG. 42 is a view showing the relationship between strength and theanisotropy of formability of the hot-rolled steel sheet of theembodiment and the comparative steel.

FIG. 43 is a view showing the relationship between strength and holeexpanding properties of the cold-rolled steel sheet of the embodimentand the comparative steel.

FIG. 44 is a view showing the relationship between strength and bendingproperties of the cold-rolled steel sheet of the embodiment and thecomparative steel.

FIG. 45 is a view showing the relationship between strength and theanisotropy of formability of the cold-rolled steel sheet of theembodiment and the comparative steel.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described indetail.

1. Regarding a Hot-Rolled Steel Sheet

-   -   (1) An average value of the X-ray random intensity ratio of a        {100}<011> to {223}<110> orientation group in a sheet thickness        central portion that is in a sheet thickness range of ⅝ to ⅜        from the surface of a steel sheet, an X-ray random intensity        ratio of a {332}<113> crystal orientation:

The average value of the X-ray random intensity ratio of a {100}<011> to{223}<110> orientation group in a sheet thickness central portion thatis in a sheet thickness range of ⅝ to ⅜ from the surface of the steelsheet is a particularly important characteristic value of theembodiment.

As shown in FIG. 1, if the average value of the {100}<011> to {223}<110>orientation group is 6.0 or less when X-ray diffraction is carried outon a sheet surface in the sheet thickness central portion that is in asheet thickness range of ⅝ to ⅜ from the surface of the steel sheet sothat the intensity ratios of the respective orientations with respect toa random specimen are obtained, d/Rm which is a sheet thickness/minimumbending radius necessary for working of underbody components or skeletoncomponents is 1.5 or more. Furthermore, in a case in which holeexpanding properties or small limit bending characteristic is required,d/Rm is desirably 4.0 or less, and more desirably less than 3.0. Whend/Rm is more than 6.0, the anisotropy of the mechanical characteristicsof the steel sheet becomes extremely strong, and, consequently, evenwhen local deformability in a certain direction improves, materialqualities in directions different from the above direction significantlydegrade, and therefore it becomes impossible for the sheetthickness/minimum bending radius to be greater than or equal to 1.5. Ina case in which a cold-rolled steel sheet or hot-rolled strip which is astarting sheet for a galvanized steel sheet is used, the X-ray randomintensity ratio is preferably less than 4.0.

Meanwhile, while it is difficult to realize in a current ordinarycontinuous hot rolling process, when the X-ray random intensity ratiobecomes less than 1.0, there is a concern that local deformability maydegrade.

Furthermore, due to the same reason, if the X-ray random intensity ratioof the {332}<113> crystal orientation in the sheet thickness centralportion that is in a sheet thickness range of ⅝ to ⅜ from the surface ofthe steel sheet is 5.0 or less as shown in FIG. 2, the sheetthickness/minimum bending radius necessary for working of underbodycomponents is 1.5 or more. The sheet thickness/minimum bending radius ismore desirably 3.0 or less. When the sheet thickness/minimum bendingradius is more than 5.0, the anisotropy of the mechanicalcharacteristics of the steel sheet becomes extremely strong, and,consequently, even when local deformability improves only in a certaindirection, material qualities in directions different from the abovedirection significantly degrade, and therefore it becomes impossible forthe sheet thickness/minimum bending radius to be greater than or equalto 1.5. Meanwhile, while it is difficult to realize in a currentordinary continuous hot rolling process, when the X-ray random intensityratio becomes less than 1.0, there is a concern that the localdeformability may degrade.

The reason is not absolutely evident why the X-ray random intensityratio of the above crystal orientation is important for shape freezingproperties during bending working, but it is assumed that the X-rayrandom intensity ratio of the crystal orientation has a relationshipwith the slip behavior of crystals during bending working.

(2) rC which is the r value in the direction perpendicular to therolling direction:

rC is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable hole expanding properties orbending properties cannot always be obtained even when only the X-rayrandom intensity ratios of the above variety of crystal orientations areappropriate. As shown in FIG. 3, in addition to the X-ray randomintensity ratio, rC should be 0.70 or more.

When the upper limit of rC is set to 1.10, more favorable localdeformability can be obtained.

(3) r30 which is the r value in the direction that forms an angle of 30°with respect to the rolling direction:

r30 is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable local deformability cannotbe always obtained even when only the X-ray random intensity ratios ofthe above variety of crystal orientations are appropriate. As shown inFIG. 4, in addition to the X-ray random intensity ratio, r30 should be1.10 or less.

When the lower limit of r30 is set to 0.70, more favorable localdeformability can be obtained.

(4) rL which is the r value in the rolling direction and r60 which isthe r value in the direction that forms an angle of 60° with respect tothe rolling direction:

Furthermore, as a result of thorough studies, the inventors found that,in addition to the X-ray random intensity ratios of the above variety ofcrystal orientations, rC, and r30, when, furthermore, rL in the rollingdirection is 0.70 or more, and r60 which is the r value in the directionthat forms an angle of 60° with respect to the rolling direction is 1.10or less as shown in FIGS. 5 and 6, the sheet thickness/minimum bendingradius 2.0 is satisfied.

When the rL value and the r60 value are set to 1.10 or less and 0.70 ormore, respectively, more favorable local deformability can be obtained.

Meanwhile, generally, it is known that there is a correlation between atexture and the r value, but in the hot-rolled steel sheet according tothe embodiment, the limitation on the X-ray intensity ratio of thecrystal orientation and the limitation on the r value are not identicalto each other, and favorable local deformability cannot be obtained aslong as both limitations are satisfied at the same time.

(5) dL/dt ratios of bainite, martensite, pearlite, and austenite grains:

As a result of further investigating local deformability, the inventorsfound that, when the texture and the r value are satisfied, and furtherthe equiaxed properties of crystal grains are excellent, the directiondependency of bending working almost disappears. As an index thatindicates the equiaxed properties, the fraction of grains that have adL/dt which is a ratio of dL which is the length of crystal grains inthe structure in the hot-rolling direction to dt which is the length inthe sheet thickness direction of 3.0 or less, and are excellent in termsof equiaxed properties is 50% to 100% in the crystal grains. When thefraction is less than 50%, bending properties R in an L direction whichis the rolling direction or a C direction which is the directionperpendicular to the rolling direction degrade.

The respective structures can be determined as follows.

Pearlite is specified through structure observation using an opticalmicroscope. Next, a crystal structure is determined using an electronback scattering diffraction (EBSD), and a crystal having an fccstructure is determined to be austenite. Ferrite, bainite, andmartensite having a bcc structure can be recognized through KernelAverage Misorientation with which EBSP-OIM™ is equipped, that is,through a KAM method. In the KAM method, among measurement data, theorientation differences of 6 closest pixels of a regular hexagonalpixel, of 12 second closest pixels outside the closest pixels, or of 18third closest pixels outside the second closest pixels are averaged, anda value is computed by carrying out calculation in which the averagedvalue is used as the value of the central pixel on the respectivepixels. A map that represents an orientation change in a grain can beprepared by carrying out the calculation within grain boundaries. Themap represents a distribution of strain based on the local orientationchange in the grain.

In the examples of the present invention, as a condition under which theorientation difference between adjacent pixels in EBSP-OIM™ iscalculated, the orientation difference was set to 5° or less withrespect to the third closest pixel, and a pixel having an orientationdifference with respect to the third closet pixel of more than 1° wasdefined as bainite or martensite which is a product of low-temperaturetransformation, and a pixel having an orientation difference withrespect to the third closet pixel of 1° or less was defined as ferrite.This is because polygonal pro-eutectic ferrite transformed at a hightemperature is generated through diffusion transformation, and thereforethe dislocation density is small, and strain in the grain is small sothat the difference of crystal orientations in the grain is small, andthe ferrite volume fraction obtained from a variety of investigationsthat the inventors have carried out using optical microscope observationand the area fraction obtained at an orientation difference with respectto a third closest pixel of 1° measured through the KAM method,approximately match.

(6) Fraction of crystal grains having a grain diameter of more than 20μm:

Furthermore, it was found that the bending properties are stronglyinfluenced by the equiaxed properties of crystal grains, and the effectis large. The reasons are not evident, but it is considered that a modeof bending deformation is a mode in which strain locally concentrates,and a state in which all crystal grains are uniformly and equivalentlystrained is advantageous for bending properties. It is considered that,in a case in which there are many crystal grains having a large graindiameter, even when crystal grains are sufficiently made to be isotropicand equiaxed, crystal grains locally strain, and a large variationappears in the bending properties due to the orientation of the locallystrained crystal grains such that degradation of the bending propertiesis caused. Therefore, in order to suppress localization of strain andimprove the bending properties by the effect of being made isotropic andequiaxed, the area fraction of crystal grains having a grain diameter ofmore than 20 μm is preferably smaller, and needs to be 0% to 10%. Whenthe area fraction is larger than 10%, the bending propertiesdeteriorate. The crystal grains mentioned herein refer to crystal grainsof ferrite, pearlite, bainite, martensite, and austenite.

The present invention is generally applicable to hot-rolled steelsheets, and, as long as the above limitations are satisfied, localdeformability, such as the bending workability or hole expandingproperties of a hot-rolled steel sheet, drastically improves without thelimitation on combination of structures.

2. Regarding a Cold-Rolled Steel Sheet

(1) An average value of the X-ray random intensity ratio of a {100}<011>to {223}<110> orientation group in a sheet thickness central portionthat is in a sheet thickness range of ⅝ to ⅜ from the surface of a steelsheet, and an X-ray random intensity ratio of a {332}<113> crystalorientation:

The average value of the X-ray random intensity ratio of a {100}<011> to{223}<110> orientation group in a sheet thickness central portion thatis in a sheet thickness range of ⅝ to ⅜ from the surface of the steelsheet is particularly important the embodiment.

As shown in FIG. 7, if the average value of the {100}<011> to {223}<110>orientation group is less than 4.0 when an X-ray diffraction is carriedout on a sheet surface in the sheet thickness central portion that is ina sheet thickness range of ⅝ to ⅜ from the surface of the steel sheet sothat the intensity ratios of the respective orientations with respect toa random specimen are obtained, a sheet thickness/minimum bending radiusnecessary for working of skeleton components is 1.5 or more.Furthermore, in a case in which hole expanding properties or a smalllimit bending characteristic is required, the sheet thickness/minimumbending radius is desirably less than 3.0. When the sheetthickness/minimum bending radius is 4.0 or more, the anisotropy of themechanical characteristics of the steel sheet becomes extremely strong,and, consequently, even when local deformability in a certain directionimproves, material qualities in directions different from the abovedirection significantly degrade, and therefore it becomes impossible forthe sheet thickness/minimum bending radius to be greater than or equalto 1.5.

Meanwhile, while it is difficult to realize in a current ordinarycontinuous hot rolling process, when the X-ray random intensity ratiobecomes less than 1.0, there is a concern that local deformability maydegrade.

Furthermore, due to the same reason, if the X-ray random intensity ratioof the {332}<113> crystal orientation in the sheet thickness centralportion that is in a sheet thickness range of ⅝ to ⅜ from the surface ofthe steel sheet is 5.0 or less as shown in FIG. 8, the sheetthickness/minimum bending radius necessary for working of skeletoncomponents is 1.5 or more. The sheet thickness/minimum bending radius ismore desirably 3.0 or less. When the sheet thickness/minimum bendingradius is more than 5.0, the anisotropy of the mechanicalcharacteristics of the steel sheet becomes extremely strong, and,consequently, even when local deformability improves only in a certaindirection, material qualities in directions different from the abovedirection significantly degrade, and therefore it becomes impossible forthe sheet thickness/minimum bending radius to be greater than or equalto 1.5. Meanwhile, while it is difficult to realize in a currentordinary continuous hot rolling process, when the X-ray random intensityratio becomes less than 1.0, there is a concern that local deformabilitymay degrade.

The reason is not absolutely evident why the X-ray random intensityratio of the above crystal orientation is important for shape freezingproperties during bending working, but it is assumed that the X-rayrandom intensity ratio of the crystal orientation has a relationshipwith the slip behavior of crystals during bending working.

(2) rC which is the r value in the direction perpendicular to therolling direction:

rC is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable hole expanding properties orbending properties cannot be always obtained even when only the X-rayrandom intensity ratios of the above variety of crystal orientations areappropriate. As shown in FIG. 9, in addition to the X-ray randomintensity ratio, rC should be 0.70 or more.

When the upper limit of rC is set to 1.10, more favorable localdeformability can be obtained.

(3) r30 which is the r value in the direction that forms an angle of 30°with respect to the rolling direction:

r30 is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable local deformability cannotbe always obtained even when only the X-ray random intensity ratios ofthe above variety of crystal orientations are appropriate. As shown inFIG. 10, in addition to the X-ray random intensity ratio, r30 should be1.10 or less.

When the lower limit of r30 is set to 0.70, more favorable localdeformability can be obtained.

(4) rL which is the r value in the rolling direction and r60 which isthe r value in the direction that forms an angle of 60° with respect tothe rolling direction:

Furthermore, as a result of thorough studies, the inventors found that,in addition to the X-ray random intensity ratios of the above variety ofcrystal orientations, rC, and r30, when, furthermore, rL in the rollingdirection is 0.70 or more, and r60 which is the r value in the directionthat forms an angle of 60° with respect to the rolling direction is 1.10or less as shown in FIGS. 11 and 12, the sheet thickness/minimum bendingradius is equal to or greater than 2.0.

When the rL and the r60 are set to 1.10 or less and 0.70 or morerespectively, more a favorable local deformability can be obtained.

Meanwhile, generally, it is known that there is a correlation between atexture and the r value, in the cold-rolled steel sheet according to theembodiment, the limitation on the X-ray intensity ratio of the crystalorientation and the limitation on the r value are not identical to eachother, and favorable local deformability cannot be obtained as long asboth limitations are satisfied at the same time.

(5) dL/dt ratios of bainite, martensite, pearlite, and austenite grains:

As a result of further investigating local deformability, the inventorsfound that, when the texture and the r value are satisfied, and furtherthe equiaxed properties of crystal grains are excellent, the directiondependency of bending working almost disappears. As an index thatindicates the equiaxed properties, it is important that the fraction ofgrains that have a dL/dt, which is a ratio of dL which is the length ofcrystal grains in the structure in the cold-rolling direction to dtwhich is the length in the sheet thickness direction, of 3.0 or less,and are excellent in terms of equiaxed properties is 50% to 100% in thecrystal grains. When the fraction is less than 50%, bending properties Rin an L direction which is the rolling direction or in a C directionwhich is the direction perpendicular to the rolling direction degrade.

The respective structures can be determined as follows.

Pearlite is specified through structure observation using an opticalmicroscope. Next, a crystal structure is determined using electron backscattering diffraction (EBSD), and a crystal having an fcc structure isdetermined to be austenite. Ferrite, bainite, and martensite having abcc structure can be recognized through Kernel Average Misorientationwith which EBSP-OIM™ is equipped, that is, through a KAM method. In theKAM method, among measurement data, the orientation differences of 6closest pixels of a regular hexagonal pixel, of 12 second closest pixelsoutside the closest pixels, or of 18 third closest pixels outside thesecond closest pixels are averaged, and a value is computed by carryingout calculation in which the averaged value is used as the value of thecentral pixel on the respective pixels. A map that represents anorientation change in a grain can be prepared by carrying out thecalculation within grain boundaries. The map represents a distributionof strain based on the local orientation change in the grain.

In the examples of the present invention, as a condition under which theorientation difference between adjacent pixels in EBSP-OIM™, theorientation difference was set to 5° or less with respect to the thirdclosest pixel, and a pixel having an orientation difference with respectto the third closet pixel of more than 1° was defined as bainite ormartensite which is a product of low-temperature transformation, and apixel having an orientation difference with respect to the third closetpixel of 1° or less was defined as ferrite. This is because polygonalpro-eutectic ferrite transformed at a high temperature is generatedthrough diffusion transformation, and therefore the dislocation densityis small, and strain in the grain is small so that the difference ofcrystal orientations in the grain is small, and the ferrite volumefraction obtained from a variety of investigations that the inventorshave carried out using optical microscope observation and the areafraction obtained at an orientation difference third closest pixel of 1°measured through the KAM method approximately match.

(6) Fraction of crystal grains having a grain diameter of more than 20μm:

Furthermore, it was found that the bending properties are stronglyinfluenced by the equiaxed properties of crystal grains, and the effectis large. The reasons are not evident, but it is considered that bendingdeformation is a mode in which strain locally concentrates, and a statein which all crystal grains are uniformly and equivalently strained isadvantageous for bending properties. It is considered that, in a case inwhich there are many crystal grains having a large grain diameter, evenwhen crystal grains are sufficiently made to be isotropic and equiaxed,crystal grains locally strain, and a large variation appears in thebending properties due to the orientation of the locally strainedcrystal grains such that degradation in the bending properties iscaused. Therefore, in order to suppress localization of strain andimprove the bending properties through the effect of making isotropicand equiaxed, the area fraction of crystal grains having a graindiameter of more than 20 μm is preferably smaller, and needs to be 0% to10%. When the area fraction is larger than 10%, the bending propertiesdeteriorate. The crystal grains mentioned herein refer to crystal grainsof ferrite, pearlite, bainite, martensite, and austenite.

The present invention is generally applicable to cold-rolled steelsheets, and, as long as the above limitations are satisfied, localdeformability, such as the bending workability or hole expandingproperties of a cold-rolled steel sheet, drastically improves withoutlimitation on combination of structures.

3. Regarding a Galvanized Steel Sheet

(1) An average value of the X-ray random intensity ratio of a {100}<011>to {223}<110> orientation group in a sheet thickness central portionthat is in a sheet thickness range of ⅝ to ⅜ from the surface of a steelsheet, an X-ray random intensity ratio of a {332}<113> crystalorientation:

The average value of the X-ray random intensity ratio of a {100}<011> to{223}<110> orientation group in a sheet thickness central portion thatis in a sheet thickness range of ⅝ to ⅜ from the surface of the steelsheet is particularly important in the embodiment. As shown in FIG. 13,if the average value of the {100}<011> to {223}<110> orientation groupis less than 4.0 when an X-ray diffraction is carried out on a sheetsurface in the sheet thickness central portion that is in a sheetthickness range of ⅝ to ⅜ from the surface of the steel sheet so thatthe intensity ratios of the respective orientations with respect to arandom specimen are obtained, a sheet thickness/minimum bending radiusnecessary for working of underbody components or skeleton components is1.5 or more. Furthermore, in a case in which hole expanding propertiesor a small limit bending characteristic is required, the sheetthickness/minimum bending radius is desirably less than 3.0. When thesheet thickness/minimum bending radius is 4.0 or more, the anisotropy ofthe mechanical characteristics of the steel sheet becomes extremelystrong, and, consequently, even when local deformability in a certaindirection improves, material qualities in directions different from theabove direction significantly degrade, and therefore it becomesimpossible for the sheet thickness/minimum bending radius to be greaterthan or equal to 1.5.

Meanwhile, while it is difficult to realize in a current ordinarycontinuous hot rolling process, when the X-ray random intensity ratiobecomes less than 1.0, there is a concern that local deformability maydegrade.

Furthermore, due to the same reason, if the X-ray random intensity ratioof the {332}<113> crystal orientation in the sheet thickness centralportion that is in a sheet thickness range of ⅝ to ⅜ from the surface ofthe steel sheet is 5.0 or less as shown in FIG. 14, the sheetthickness/minimum bending radius necessary for working of underbodycomponents is 1.5 or more. The sheet thickness/minimum bending radius ismore desirably 3.0 or less. When the sheet thickness/minimum bendingradius is more than 5.0, the anisotropy of the mechanicalcharacteristics of the steel sheet becomes extremely strong, and,consequently, even when local deformability improves only in a certaindirection, material qualities in directions different from the abovedirection significantly degrade, and therefore it becomes impossible toreliably satisfy the sheet thickness/minimum bending radius 1.5.Meanwhile, while it is difficult to realize in a current ordinarycontinuous hot rolling process, when the X-ray random intensity ratiobecomes less than 1.0, there is a concern that local deformability maydegrade.

The reason is not absolutely evident why the X-ray random intensityratio of the above crystal orientation is important for shape freezingproperties during bending working, but it is assumed that the X-rayrandom intensity ratio of the crystal orientation has a relationshipwith the slip behavior of crystals during bending working.

rC which is the r value in the direction perpendicular to the rollingdirection:

rC is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable hole expanding properties orbending properties cannot be always obtained even when only the X-rayrandom intensity ratios of the above variety of crystal orientations areappropriate. As shown in FIG. 15, in addition to the X-ray randomintensity ratio, rC should be 0.70 or more.

When the upper limit of rC is set to 1.10, more favorable localdeformability can be obtained.

r30 which is the r value in the direction that forms an angle of 30°with respect to the rolling direction:

r30 is important in the embodiment. That is, as a result of thoroughstudies, the inventors found that favorable hole expanding properties orbending properties cannot be always obtained even when only the X-rayrandom intensity ratios of the above variety of crystal orientations areappropriate. As shown in FIG. 16, in addition to the X-ray randomintensity ratio, r30 should be 1.10 or less.

When the lower limit of r30 is set to 0.70, more favorable localdeformability can be obtained.

rL which is the r value in the rolling direction, and r60 which is the rvalue in the direction that forms an angle of 60° with respect to therolling direction:

Furthermore, as a result of thorough studies, the inventors found that,in addition to the X-ray random intensity ratios of the above variety ofcrystal orientations, rC, and r30, when, furthermore, rL in the rollingdirection is 0.70 or more, and r60 which is the r value in the directionthat forms an angle of 60° with respect to the rolling direction is 1.10or less as shown in FIGS. 17 and 18, the sheet thickness/minimum bendingradius will be greater than or equal to 2.0.

When the rL value and the r60 value are set to 1.10 or less and 0.70 ormore, respectively, more favorable local deformability can be obtained.

Meanwhile, generally, it is known that there is a correlation between atexture and the r value, in the galvanized steel sheet according to thepresent invention, the limitation on the X-ray intensity ratio of thecrystal orientation and the limitation on the r value are not identicalto each other, and favorable local deformability cannot be obtained aslong as both limitations are not satisfied at the same time.

The present invention is generally applicable to galvanized steelsheets, and, as long as the above limitations are satisfied, localdeformability, such as the bending workability or hole expandingproperties of a galvanized steel sheet, drastically improves withoutlimitation on a combination of structures.

Main orientations included in the {100}<011> to {223}<110> orientationgroup are {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>,{335}<110>, and {223}<110>.

The X-ray random intensity ratios of the respective orientations can bemeasured using a method, such as X-ray diffraction or electron backscattering diffraction (EBSD). Specifically, the X-ray random intensitymay be obtained from a 3-dimensional texture computed through a vectormethod based on the {110} pole figure or a 3-dimensional texturecomputed through a series expansion method using a plurality of polefigures (preferably three or more) among {110}, {100}, {211}, and {310}pole figures.

For example, as the X-ray random intensity ratios of the respectivecrystal orientations in the EBSD method, the intensities of (001)[1-10], (116) [1-10], (114) [1-10], (113) [1-10], (112) [1-10], (335)[1-10], and (223) [1-10] in a φ2=45° cross section of a 3-dimensionaltexture may be used as they are. The 1 with bar above which indicatesnegative 1 is expressed by −1.

In addition, the average value of the {100}<011> to {223}<110>orientation group is the arithmetic average of the respectiveorientations. In a case in which the intensities of all of the aboveorientations cannot be obtained, the intensities may be replaced withthe arithmetic average of the respective orientations of {100}<011>,{116}<110>, {114}<110>, {112}<110>, and {223}<110>.

For measurement, a specimen provided for X-ray diffraction or EBSD issubjected to mechanical polishing or the like so that the steel sheet isreduced from the surface to be a predetermined sheet thickness, next,strain is removed through chemical polishing or electrolytic polishing,and, at the same time, the specimen is adjusted through the above methodso that an appropriate surface in a sheet thickness range of ⅝ to ⅜becomes a measurement surface. The specimen is desirably taken from alocation of a ¼ or ¾ width from the end portion in the sheet widthdirection.

It is needless to say that, when the limitation on the X-ray intensityis satisfied not only at the vicinity of ½ of the sheet thickness butalso at as many thicknesses as possible, local deformability becomesmore favorable. However, since, generally, the material characteristicsof the entire steel sheet can be represented by measuring the sheetthickness central portion that is in a sheet thickness range of ⅝ to ⅜from the surface of the steel sheet, the average value of the X-rayrandom intensity ratios of the {100}<011> to {22}<110> orientation groupin the sheet thickness central portion that is in a sheet thicknessrange of ⅝ to ⅜ from the surface of the steel sheet and the X-ray randomintensity ratio of the {332}<113> crystal orientation are specified. Thecrystal orientation that is represented by {hkl}<uvw> indicates that thenormal direction of the sheet surface is parallel with {hkl}, and therolling direction is parallel to <uvw>.

In addition, the respective r values are evaluated through tensile testsin which JIS No. 5 tensile test specimens are used. In the case of ahigh-strength steel sheet, tensile strain may be evaluated in a range of5% to 15% using a range of uniform elongation.

Since a direction in which bending working is carried out varies bycomponents to be worked, the direction is not particularly limited;however, according to the present invention, the same characteristicscan be obtained in all bending directions.

The dL/dt and grain diameter of pearlite can be obtained through abinarization and a point counter method in structure observation usingan optical microscope.

In addition, the grain diameters of ferrite, bainite, martensite, andaustenite can be obtained by measuring orientations, for example, at amagnification of 1500 times and a measurement step (pitch) of 0.5 μm orless in an analysis of steel sheet orientations through the EBSD method,specifying locations at which the orientation difference betweenadjacent measurement points exceeds 15° as grain boundaries, andobtaining a diameter of the equivalent circle. At this time, the lengthsof a grain in the rolling direction and the sheet thickness directionare obtained at the same time, thereby obtaining dL/dt.

Next, conditions for limiting the steel sheet components will bedescribed. % for contents is mass %.

Since the cold-rolled steel sheet and galvanized steel sheet of thepresent invention use the hot-rolled steel sheet of the presentinvention as a raw sheet, the components of a steel sheet will be asfollows for all of the hot-rolled steel sheet, the cold-rolled steelsheet, and the galvanized steel sheet.

C is a basically included element, and the reason why the lower limit isset to 0.0001% is to use the lower limit value obtained from practicalsteel. When the upper limit exceeds 0.40%, workability or weldabilitydeteriorates, and therefore the upper limit is set to 0.40%. Meanwhile,since excessive addition of C significantly deteriorates spotweldability, the upper limit is more desirably set to 0.30% or lower.

Si is an effective element for enhancing the mechanical strength of asteel sheet, and, when the content exceeds 2.5%, workabilitydeteriorates, or surface defects are generated, and therefore the upperlimit is set to 2.5%. On the other hand, since it is difficult toinclude Si at less than 0.001% in practical steel, the lower limit isset to 0.001%.

Mn is an effective element for enhancing the mechanical strength of asteel sheet, and, when the content exceeds 4.0%, the workabilitydeteriorates, and therefore the upper limit is set to 4.0%. On the otherhand, since it is difficult to include Mn at less than 0.001% inpractical steel, the lower limit is set to 0.001%. However, in order toavoid an extreme increase in steel-manufacturing costs, the lower limitis desirably set to 0.01% or more. Since Mn suppresses generation offerrite, in a case in which it is intended to include a ferrite phase ina structure so as to secure elongation, the lower limit is desirably setto 3.0% or less. In addition, in a case in which, other than Mn,elements which suppress generation of hot cracking caused by S, such asTi, are not added, Mn is desirably added at an amount so that Mn/Sbecomes equal to or larger than 20 in terms of mass %.

The upper limits of P and S are 0.15% or less and 0.03% or lessrespectively in order to prevent deterioration of workability orcracking during hot rolling or cold rolling. The respective lower limitsare set to 0.001% for P and 0.0005% for S which are values obtainablethrough current ordinary purification (including secondarypurification). Meanwhile, since extreme desulfurization significantlyincreases the costs, the lower limit of S is more desirably 0.001% ormore.

For deoxidizing, Al is added at 0.001% or more. However, in a case inwhich sufficient deoxidizing is required, Al is more desirably added at0.01% or more. In addition, since Al significantly increases the γ→αtransformation point from γ to α, Al is an effective element in a casein which hot rolling particularly at Ar3 point or lower is oriented.However, when Al is excessive, weldability deteriorates, and thereforethe upper limit is set to 2.0%.

N and O are impurities, and are both set to 0.01% or less so as toprevent workability from degrading. The lower limits are set to 0.0005%which is a value obtainable through current ordinary purification(including secondary purification) for both elements. However, thecontents of N and O are desirably set to 0.001% or more in order tosuppress an extreme increase in steel-manufacturing costs.

Furthermore, in order to enhance the mechanical strength throughprecipitation strengthening, or to control inclusions or refineprecipitates for improving local deformability, the steel sheet maycontain one or two or more of any of Ti, Nb, B, Mg, REM, Ca, Mo, Cr, V,W, Cu, Ni, Co, Sn, Zr, and As which have been thus far used. In order toachieve precipitation strengthening, it is effective to generate finecarbonitrides, and addition of Ti, Nb, V, or W is effective. Inaddition, Ti, Nb, V, and W also have an effect of contributing torefinement of crystal grains as solid solution elements.

In order to obtain the effect of precipitation strengthening throughaddition of Ti, Nb, V, or W, it is necessary to add 0.001% or more ofTi, 0.001% or more of Nb, 0.001% or more of V, or 0.001% or more of W.In a case in which precipitation strengthening is particularly required,it is more desirable to add 0.01% or more of Ti, 0.005% or more of Nb,0.01% or more of V, or 0.01% or more of W. Furthermore, Ti and Nb havean effect of improving material quality through mechanisms of fixationof carbon and nitrogen, structure control, fine grain strengthening, andthe like in addition to precipitate strengthening. In addition, V iseffective for precipitation strengthening, causes less degradation oflocal deformability induced from strengthening due to addition than Moor Cr, and an effective addition element in a case in which a highstrength and better hole expanding properties or bending properties arerequired. However, even when the above elements are excessively added,since the effect of an increase in strength is saturated, and,furthermore, recrystallization after hot rolling is suppressed such thatit is difficult to control crystal orientation, it is necessary to addTi and Nb at 0.20% or less and V and W at 1.0% or less. However, in acase in which elongation is particularly required, it is more desirableto include V at 0.50% or less and W at 0.50% or less.

In a case in which the hardenability of a structure is enhanced, and asecond phase is controlled so as to secure strength, it is effective toadd one or two or more of B, Mo, Cr, Cu, Ni, Co, Sn, Zr, and As.Furthermore, in addition to the above effect, B has an effect ofimproving material quality through mechanisms of fixation of carbon andnitrogen, structure control, fine grain strengthening, and the like. Inaddition, in addition to the effect of enhancing the mechanicalstrength, Mo and Cr have an effect of improving material quality.

In order to obtain the above effects, it is necessary to add B at0.0001% or more, Mo, Cr, Ni, and Cu at 0.001% or more, and Co, Sn, Zr,and As at 0.0001% or more. However, in contrast, since excessiveaddition deteriorates workability, the upper limit of B is set to0.0050%, the upper limit of Mo is set to 1.0%, the upper limits of Cr,Ni, and Cu are set to 2.0%, the upper limit of Co is set to 1.0%, theupper limits of Sn and Zr are set to 0.2%, and the upper limit of As isset to 0.50%. In a case in which there is a strong demand forworkability, it is desirable to set the upper limit of B to 0.005% andthe upper limit of Mo to 0.50%. In addition, it is more desirable toselect B, Mo, Cr, and As among the above addition elements from theviewpoint of costs.

Mg, REM, and Ca are important addition elements that detoxify inclusionsand further improve local deformability. The lower limits for obtainingthe above effect are 0.0001% respectively; however, in a case in whichit is necessary to control the shapes of inclusions, Mg, REM, and Ca aredesirably added at 0.0005% or more respectively. Meanwhile, sinceexcessive addition results in degradation of cleanness, the upper limitsof Mg, REM, and Ca are set to 0.010%, 0.1%, and 0.010% respectively.

The effect of improving local deformability is not lost even when asurface treatment is carried out on the hot-rolled steel sheet andcold-rolled steel sheet of the present invention, and the effects of thepresent invention can be obtained even when any of electroplating, hotdipping, deposition plating, organic membrane formation, filmlaminating, an organic salts/inorganic salts treatment, non-chromiumtreatment, and the like is carried out.

In addition, the galvanized steel sheet of the present invention has agalvanized layer by carrying out a galvanizing treatment on the surfaceof the cold-rolled steel sheet of the present invention, and galvanizingcan obtain the effects both in hot dip galvanizing andelectrogalvanizing. In addition, the galvanized steel sheet of thepresent invention may be produced as a zinc alloy-plated steel sheetmainly used for automobiles by carrying out an alloying treatment aftergalvanizing.

Additionally, the effects of the present invention are not lost evenwhen a surface treatment is further carried out on the high-strengthgalvanized steel sheet of the present invention, and the effects of thepresent invention can be obtained even when any of electroplating, hotdipping, deposition plating, organic membrane formation, filmlaminating, an organic salts/inorganic salts treatment, non-chromiumtreatment, and the like is carried out.

2. Regarding the Manufacturing Method

Next, the method of manufacturing a hot-rolled steel sheet according tothe embodiment will be described.

In order to realize excellent local deformability, it is important toform a texture having a predetermined X-ray random intensity ratio,satisfy the conditions for the r values in the respective directions,and control the grain shapes. Details of the manufacturing conditionsfor satisfying the above will be described below.

A manufacturing method preceding hot rolling is not particularlylimited. That is, subsequent to ingoting using a blast furnace, anelectric furnace, or the like, a variety of secondary purifications arecarried out, then, the ingot may be cast through a method, such asordinary continuous casting, an ingot method, or thin slab casting. Inthe case of continuous casting, the ingot may be once cooled to a lowtemperature, reheated, and then hot-rolled, or a cast slab may also behot-rolled as it is after casting without cooling the cast slab to a lowtemperature. Scraps may be used as a raw material.

The hot-rolled steel sheet according to the embodiment is obtained in acase in which the following conditions are satisfied.

In order to satisfy the above predetermined values of rC of 0.70 or moreand r30 of 1.10 or less, the austenite grain diameter after roughrolling, that is, before finishing rolling is important. As shown inFIGS. 19 and 20, the austenite grain diameter before finishing rollingmay be 200 μm or less.

In order to obtain an austenite grain diameter before finishing rollingof 200 μm or less, in the rough rolling, it is necessary to carry outrolling in a temperature range of 1000° C. to 1200° C. and carry outrolling once or more at a rolling reduction ratio of at least 20% ormore in the temperature range as shown in FIG. 21. However, in order tofurther enhance homogeneity and enhance elongation and localdeformability, it is desirable to carry out rolling once or more at arolling reduction ratio of at least 40% or more in a temperature rangeof 1000° C. to 1200° C.

The austenite grain diameter is more desirably set to 100 μm or less,and, in order to achieve the austenite grain diameter of 100 μm or less,it is desirable to carry out rolling twice or more at a rollingreduction ratio of 20% or more. Desirably, rolling is carried out twiceor more at a rolling reduction ratio of 40% or more. As the rollingreduction ratio and the number of times of rolling increase, smallergrains can be obtained, but there is a concern that the temperature maydecrease or scales may be excessively generated when the rolling exceeds70% or the number of times of the rough rolling exceeds 10 times. Assuch, a decrease in the austenite grain diameter before finishingrolling is effective to improve local deformability through accelerationof recrystallization of austenite during subsequent finishing rolling,particularly through control of rL or r30.

The reason why refinement of the austenite grain diameter has aninfluence on local deformability is assumed to be that austenite grainboundaries after the rough rolling, that is, austenite grain boundariesbefore the finishing rolling function as one of recrystallization nucleiduring the finishing rolling.

In order to confirm the austenite grain diameter after the roughrolling, it is desirable to cool a sheet piece that is about to befinishing-rolled as rapidly as possible. The sheet piece is cooled at acooling rate of 10° C./s or more, the structure on the cross section ofthe sheet piece is etched, austenite grain boundaries are made toappear, and the austenite grain diameter is measured using an opticalmicroscope. At this time, the austenite grain diameter is measured at amagnification of 50 times or more at 20 sites or more through an imageanalysis or a point counter method.

In addition, in order to achieve an average value of the X-ray randomintensity ratio of the {100}<011> to {223}<110> orientation group in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from the steel sheet surface and an X-ray random intensity ratio of the{332}<113> crystal orientation in the above value ranges, based on theT1 temperature described in the formula 1 which is determined by thesteel sheet components in the finishing rolling after the rough rolling,working is carried out at a large rolling reduction ratio in atemperature range of T1+30° C. to T1+200° C., desirably in a temperaturerange of T1+50° C. to T1+100° C., and working is carried out at a smallrolling reduction ratio in a temperature range of T1° C. to lower thanT1+30° C. According to the above, the local deformability and shape of afinal hot-rolled product can be secured. FIGS. 22 to 25 show therelationships between the rolling reduction ratios in the respectivetemperature ranges and the X-ray random intensity ratios of therespective orientations.

That is, as shown in FIGS. 22 and 24, large reduction in a temperaturerange of T1+30° C. to T1+200° C. and subsequent light rolling at T1° C.to lower than T1+30° C. as shown in FIGS. 23 and 25 control the averagevalue of the X-ray random intensity ratio of the {100}<011> to{223}<110> orientation group in a thickness central portion that is in asheet thickness range of ⅝ to ⅜ from the steel sheet surface and theX-ray random intensity ratio of the {332}<113> crystal orientation so asto drastically improve the local deformability of the final hot-rolledproduct.

The T1 temperature is experimentally obtained, and the inventors foundfrom experiments that recrystallization in the austenite range of therespective steels is accelerated with the T1 temperature as a basis.

In order to obtain more favorable local deformability, it is importantto accumulate strain through the large reduction or repeatedlyrecrystallize the structure every rolling. In order to accumulatestrain, the total rolling reduction ratio is 50% or more, and desirably70% or more, and, furthermore, an increase in the temperature of thesteel sheet between passes is desirably set to 18° C. or lower.Meanwhile, the total rolling reduction of more than 90% is not desirablefrom the viewpoint of temperature securement or excessive rolling load.Furthermore, in order to enhance the homogeneity of a hot-rolled sheet,and enhance the local deformability to the extreme, among the rollingpasses in a temperature range of T1+30° C. to T1+200° C., at least onepass is carried out at a rolling reduction ratio of 30% or more, anddesirably at 40% or more. Meanwhile, when the rolling reduction ratioexceeds 70% in a pass, there is a concern that the shape may beimpaired. In a case in which there is a demand for more favorableworkability, it is more desirable to set the rolling reduction ratio to30% or more in the final 2 passes.

Furthermore, in order to accelerate uniform recrystallization throughreleasing of accumulated strain, it is necessary to suppress as much aspossible the working amount in a temperature range of T1° C. to lowerthan T1+30° C. after the large reduction at T1+30° C. to T1+200° C., andthe total rolling rate at T1° C. to lower than T1+30° C. is set to lessthan 30%. A rolling reduction ratio of 10% or more is desirable from theviewpoint of the sheet shape, but a rolling reduction ratio of 0% isdesirable in a case in which local deformability matters more. When therolling reduction ratio at T1° C. to lower than T1+30° C. exceeds apredetermined range, recrystallized austenite grains are expanded, and,when the retention time is short, recrystallization does notsufficiently proceed, and the local deformability deteriorates. That is,in the manufacturing conditions according to the embodiment, it isimportant to uniformly and finely recrystallize austenite duringfinishing rolling so as to control the texture of a hot-rolled productin order to improve local deformability, such as hole expandingproperties or bending properties.

When rolling is carried out at a lower temperature than the temperaturerange specified above or at a larger rolling reduction ratio than thespecified rolling reduction ratio, the texture of austenite develops,and the X-ray random intensity ratios in the respective crystalorientations, such as the average value of the X-ray random intensityratio of the {100}<011> to {22}<110> orientation group at least in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from a steel sheet surface of 6.0 or less and the X-ray random intensityratio of the {332}<113> crystal orientation of 5.0 or less, cannot beobtained in the finally obtained hot-rolled steel sheet.

Meanwhile, when rolling is carried out at a higher temperature than thespecified temperature range or at a smaller rolling reduction ratio thanthe specified rolling reduction ratio, grain coarsening or duplex grainsresults, and the area fraction of crystal grains having a grain diameterof larger than 20 μm increases. Whether or not the above-specifiedrolling is carried out can be determined from rolling reduction ratio,rolling load, sheet thickness measurement, or the like through actualperformance or calculation. In addition, since the temperature can bealso measured if a thermometer is present between stands, andcalculation simulation in which working heat generation and the like areconsidered from line speed, rolling reduction ratio, and the like isavailable, whether or not the above-specified rolling is carried out canbe determined using either or both of temperature and calculationsimulation.

The hot rolling carried out in the above manner ends at a temperature ofAr3 or higher. When the end temperature of the hot rolling is lower thanAr3, since two-phase region rolling in an austenite area and a ferritearea is included, accumulation into the {100}<011> to {22}<110>orientation group becomes strong, and, consequently, local deformabilitysignificantly degrades.

As long as rL and r60 are 0.70 or more and 1.10 or less respectively,furthermore, favorable sheet thickness/minimum bending radius 2.0 issatisfied. In order to achieve the sheet thickness/minimum bendingradius 2.0, in a case in which a pass in which the rolling reductionratio is 30% or more in the temperature range of T1+30° C. to T1+200° C.is defined as a large reduction pass, a waiting time t (seconds) fromcompletion of the final pass of the large reduction pass to initiationof cooling satisfies the formula 2, and the temperature increase of thesteel sheet between the respective passes is desirably 18° C. or lower.

FIGS. 26 and 27 show the relationship among the temperature increaseamount of the steel sheet between the passes during rolling at T1+30° C.to T1+200° C.; the waiting time t; and rL and r60. In a case in whichthe temperature increase of the steel sheet between the respectivepasses at T1+30° C. to T1+200° C. is 18° C. or lower, and t satisfiesthe formula 2, it is possible to obtain uniform recrystallized austenitehaving an rL of 0.70 or more and an r60 of 1.10 or less.

When the waiting time t exceeds t1×2.5, grain coarsening proceeds, andelongation significantly degrades. In addition, when the waiting time tis shorter than t1, anisotropy increases, and the equiaxed grainproportion decreases.

In a case in which the temperature increase of the steel sheet at T1+30°C. to T1+200° C. is too low to obtain a predetermined rolling reductionratio in a range of T1+30° C. to T1+200° C., recrystallization issuppressed. In addition, in a case in which the waiting time t (seconds)does not satisfy the formula 2, grains are coarsened by the time beingtoo long, recrystallization does not proceed by the time being tooshort, and sufficient local deformability cannot be obtained.

A cooling pattern after rolling is not particularly limited. The effectsof the present invention can be obtained by employing a cooling patternfor controlling the structure according to the respective objects.

During hot rolling, a sheet bar may be joined after rough rolling, andfinishing rolling may be continuously carried out. At this time, a roughbar may be once rolled into a coil shape, stored in a cover having aheat-retention function as necessary, and again rolled back, whereby therough bar is joined.

In addition, rolling may be carried out after hot rolling.

Skin pass rolling may be carried out on the hot-rolled steel sheetaccording to necessity. Skin pass rolling has an effect of preventingthe stretcher strain which occurs during working forming or flatnesscorrection.

The structure of the hot-rolled steel sheet obtained in the embodimentmainly includes ferrite, but may include pearlite, bainite, martensite,austenite, and compounds such as carbonitrides, as metallic structuresother than ferrite. Since the crystal structure of martensite or bainiteis the same as or similar to the crystal structure of ferrite, the abovestructures may be a main component instead of ferrite.

Further, the steel sheet according to the present invention can beapplied not only to bending working but also to combined formingcomposed mainly of bending, overhanging, drawing, and bending working.

Next, the method of manufacturing a cold-rolled steel sheet according tothe embodiment will be described. In order to realize excellent localdeformability, in a steel sheet that has undergone cold rolling, it isimportant to form a texture having a predetermined X-ray randomintensity ratio, satisfy the conditions of the r values in therespective directions, and control grain shapes. Details of themanufacturing conditions for satisfying the above will be describedbelow.

A manufacturing method preceding hot rolling is not particularlylimited. That is, subsequent to ingoting using a blast furnace, anelectric furnace, or the like, a variety of secondary purifications arecarried out, then, the ingot may be cast through a method, such asordinary continuous casting, an ingot method, or thin slab casting. Inthe case of continuous casting, the ingot may be once cooled to a lowtemperature, reheated, and then hot-rolled, or a cast slab may also behot-rolled as it is after casting without cooling the cast slab to a lowtemperature. Scraps may be used as a raw material.

The cold-rolled steel sheet having excellent local deformabilityaccording to the embodiment is obtained in a case in which the followingconditions are satisfied.

In order for rC and r30 to satisfy the above predetermined values, theaustenite grain diameter after rough rolling, that is, before finishingrolling is important. As shown in FIGS. 28 and 29, the austenite graindiameter before finishing rolling is desirably small, and the abovevalues are satisfied when the austenite grain diameter is 200 μm orless.

In order to obtain an austenite grain diameter before finishing rollingof 200 μm or less, as shown in FIG. 21, it is necessary to carry out therough rolling in a temperature range of 1000° C. to 1200° C. and carryout rolling once or more at a rolling reduction ratio of at least 20% ormore. As the rolling reduction ratio and the number of times of rollingincrease, smaller grains can be obtained.

The austenite grain diameter is more desirably set to 100 μm or less,and, in order to achieve the austenite grain diameter of 100 μm or less,it is desirable to carry out rolling twice or more at a rollingreduction ratio of 20% or more. Desirably, rolling is carried out twiceor more at a rolling reduction ratio of 40% or more. As the rollingreduction ratio and the number of times of rolling increase, smallergrains can be obtained, but there is a concern that the temperature maydecrease or the scales may be excessively generated when the rollingexceeds 70% or the number of times of the rough rolling exceeds 10times. As such, a decrease in the austenite grain diameter beforefinishing rolling is effective to improve local deformability throughacceleration of recrystallization of austenite during subsequentfinishing rolling, particularly through control of rL or r30.

The reason why refinement of the austenite grain diameter has aninfluence on local deformability is assumed to be that austenite grainboundaries after the rough rolling, that is, austenite grain boundariesbefore the finishing rolling, function as one of recrystallizationnuclei during the finishing rolling. In order to confirm the austenitegrain diameter after the rough rolling, it is desirable to cool a sheetpiece that is about to be finishing-rolled as rapidly as possible. Thesheet piece is cooled at a cooling rate of 10° C./s or more, thestructure on the cross section of the sheet piece is etched, austenitegrain boundaries are made to appear, and the austenite grain diameter ismeasured using an optical microscope. At this time, the austenite graindiameter is measured at a magnification of 50 times or more at 20 sitesor more through an image analysis or a point counter method.

In addition, in order to achieve an average value of the X-ray randomintensity ratio of the {100}<011> to {22}<110> orientation group in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from the steel sheet surface, and an X-ray random intensity ratio of the{332}<113> crystal orientation in the above value ranges, based on theT1 temperature determined by the steel sheet components in the finishingrolling after the rough rolling, working is carried out at a largerolling reduction ratio in a temperature range of T1+30° C. to T1+200°C., desirably in a temperature range of T1+50° C. to T1+100° C., andworking is carried out at a small rolling reduction ratio in atemperature range of T1° C. to lower than T1+30° C. According to theabove, the local deformability and shape of a final hot-rolled productcan be secured. FIGS. 30 to 31 show the relationships between therolling reduction ratios in the temperature range of T1+30° C. toT1+200° C. and the X-ray random intensity ratios of the respectiveorientations.

That is, large reduction in a temperature range of T1+30° C. to T1+200°C. and subsequent light rolling at T1° C. to lower than T1+30° C. asshown in FIGS. 30 and 31 control the average value of the X-ray randomintensity ratio of the {100}<011> to {223}<110> orientation group in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from the steel sheet surface, and the X-ray random intensity ratio ofthe {332}<113> crystal orientation so as to drastically improve thelocal deformability of the final hot-rolled product as shown in Tables 7and 8 below. The T1 temperature is experimentally obtained, and theinventors found from experiments that recrystallization in the austeniterange of the respective steels is accelerated with the T1 temperature asa basis.

Furthermore, in order to obtain more favorable local deformability, itis important to accumulate strain through the large reduction, and thetotal rolling reduction ratio is 50% or more, more desirably 60% ormore, and still more desirably 70% or more. On the other hand, a totalrolling reduction ratio exceeding 90% is not desirable from theviewpoint of temperature securement or excessive rolling loads.Furthermore, in order to enhance the homogeneity of a hot-rolled sheet,and enhance the local deformability to the extreme, among the rollingpasses in a temperature range of T1+30° C. to T1+200° C., in at leastone pass, rolling is carried out at a rolling reduction ratio of 30% ormore, and desirably at 40% or more. Meanwhile, when the rollingreduction ratio exceeds 70% in a pass, there is a concern that the shapemay be impaired. In a case in which there is a demand for more favorableworkability, it is more desirable to set the rolling reduction ratio to30% or more in the final 2 passes.

Furthermore, in order to accelerate uniform recrystallization throughreleasing of accumulated strain, it is necessary to suppress as much aspossible the working amount in a temperature range of T1° C. to lowerthan T1+30° C. after the large reduction at T1+30° C. to T1+200° C., andthe total rolling rate at T1° C. to lower than T1+30° C. is set to lessthan 30%. A rolling reduction ratio of 10% or more is desirable from theviewpoint of the sheet shape, but a rolling reduction ratio of 0% isdesirable in a case in which local deformability matters more. When therolling reduction ratio at T1° C. to lower than T1+30° C. exceeds apredetermined range, recrystallized austenite grains are expanded, and,when the retention time is short, recrystallization does notsufficiently proceed, and the local deformability deteriorates. That is,in the manufacturing conditions according to the embodiment, it isimportant to uniformly and finely recrystallize austenite duringfinishing rolling so as to control the texture of a hot-rolled productin order to improve local deformability, such as hole expandingproperties or bending properties.

When rolling is carried out at a lower temperature than the temperaturerange specified above or at a larger rolling reduction ratio than thespecified rolling reduction ratio, the texture of austenite develops,and the X-ray random intensity ratios in the respective crystalorientations, such as the average value of the X-ray random intensityratio of the {100}<011> to {223}<110> orientation group at least in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from a steel sheet surface of less than 4.0 and the X-ray randomintensity ratio of the {332}<113> crystal orientation of 5.0 or less,cannot be obtained in the finally obtained cold-rolled steel sheet.

Meanwhile, when rolling is carried out at a higher temperature than thespecified temperature range or at a smaller rolling reduction ratio thanthe specified rolling reduction ratio, grain coarsening or duplex grainsresults, and the area fraction of crystal grains having a grain diameterof larger than 20 μm increases. Whether or not the above-specifiedrolling is carried out can be determined from the rolling reductionratio, rolling load, sheet thickness measurement, or the like throughactual performance or calculation. In addition, since the temperaturecan also be measured if a thermometer is present between stands, andcalculation simulation in which working heat generation and the like areconsidered from line speed, rolling reduction ratio, and the like isavailable, whether or not the above-specified rolling is carried out canbe determined using either or both of temperature and calculationsimulation.

The hot rolling carried out in the above manner ends at a temperature ofAr3 or higher. When the end temperature of the hot rolling is lower thanAr3, since two-phase region rolling in an austenite area and a ferritearea is included, accumulation into the {100}<011> to {223}<110>orientation group becomes strong, and, consequently, local deformabilitysignificantly degrades.

As long as rL and r60 are 0.70 or more and 1.10 or less respectively,furthermore, favorable sheet thickness/minimum bending radius is greaterthan or equal to 2.0 is satisfied. In order to achieve the sheetthickness/minimum bending radius of greater than or equal to 2.0, thetemperature increase of the steel sheet between the respective and it isdesirable to employ cooling between stands, or the like.

Furthermore, cooling after rolling at the final rolling stand of rollingmill in a temperature range of T1+30° C. to T1+200° C. has a stronginfluence on the grain diameter of austenite, which has a stronginfluence on the equiaxed grain proportion and coarse grain proportionof a cold-rolled and annealed structure. Therefore, in a case in which apass in which a rolling reduction ratio is 30% or more in a temperaturerange of T1+30° C. to T1+200° C. is defined as a large reduction pass,it is necessary for the waiting time t from completion of the final passof the large reduction pass to initiation of cooling to satisfy theformula 4. When the time being too long, grains are coarsened andelongation significantly degrades. When the time being too short,recrystallization does not proceed and sufficient local deformabilitycannot be obtained. Therefore, it is not possible for the sheetthickness/minimum bending radius is greater than or equal to 2.0.

In addition, a cooling pattern after hot rolling is not particularlyspecified, and the effects of the present invention can be obtained byemploying a cooling pattern for controlling the structure according tothe respective objects.

During hot rolling, a sheet bar may be joined after rough rolling, andfinishing rolling may be continuously carried out. At this time, a roughbar may be once rolled into a coil shape, stored in a cover having aheat-retention function as necessary, and again rolled back, whereby therough bar is joined.

On the steel sheet for which the hot rolling has been completed, coldrolling is carried out at a rolling reduction ratio of 20% to 90%. At arolling reduction ratio of less than 20%, it becomes difficult to causerecrystallization in a subsequent annealing process, and annealedcrystal grains are coarsened and the equiaxed grain proportiondecreases. At a rolling reduction ratio of more than 90%, since atexture develops during annealing, anisotropy becomes strong. Therefore,the rolling reduction ratio is set to 20% to 90% of cold rolling.

The cold-rolled steel sheet is, then, held in a temperature range of720° C. to 900° C. for 1 second to 300 seconds. When the temperature isless than 720° C. or the holding time is less than 1 second, reversetransformation does not proceed sufficiently at a low temperature or fora short time, and a second phase cannot be obtained in a subsequentcooling process, and therefore a sufficient strength cannot be obtained.On the other hand, when the temperature exceeds 900° C. or thecold-rolled steel sheet is held for 300 seconds or more, crystal grainscoarsen, and therefore the area fraction of crystal grains having agrain diameter of 20 μM or less increases. After that, the temperatureis decreased to 500° C. or less at a cooling rate of 10° C./s to 200°C./s from 650° C. to 500° C. When the cooling rate is less than 10° C./sor the cooling ends at higher than 500° C., pearlite is generated, andtherefore local deformability degrades. On the other hand, even when thecooling rate is set to more than 200° C./s, the effect of suppressingpearlite is saturated, and, conversely, the controllability of thecooling end temperature significantly deteriorates, and therefore thecooling rate is set to 200° C./s or less.

The structure of the cold-rolled steel sheet obtained in the embodimentincludes ferrite, but may include pearlite, bainite, martensite,austenite, and compounds such as carbonitrides, as metallic structuresother than ferrite. However, since pearlite deteriorates localdeformability, the content of pearlite is desirably 5% or less. Sincethe crystal structure of martensite or bainite is the same as or similarto the crystal structure of ferrite, the structure may mainly includeany of ferrite, bainite, and martensite.

Further, the cold-rolled steel sheet according to the present inventioncan be applied not only to bending working but also to combined formingcomposed mainly of bending, overhanging, drawing, and bending working.

Next, the method of manufacturing a galvanized steel sheet according tothe embodiment will be described.

In order to realize excellent local deformability, in a steel sheet thathas undergone a galvanizing treatment, it is important to form a texturehaving a predetermined X-ray random intensity ratio, satisfy theconditions of the r values in the respective directions. Details of themanufacturing conditions for satisfying the above will be describedbelow.

A manufacturing method preceding hot rolling is not particularlylimited. That is, subsequent to ingoting using a blast furnace, anelectric furnace, or the like, a variety of secondary purifications arecarried out, then, the ingot may be cast through a method, such asordinary continuous casting, an ingot method, or thin slab casting. Inthe case of continuous casting, the ingot may be once cooled to a lowtemperature, reheated, and then hot-rolled, or a cast slab may also behot-rolled as it is after casting without cooling the cast slab to a lowtemperature. Scraps may be used as a raw material.

The galvanized steel sheet having excellent local deformabilityaccording to the embodiment is obtained in a case in which the followingconditions are satisfied.

Firstly, in order for rC and r30 to satisfy the above predeterminedvalues, the austenite grain diameter after rough rolling, that is,before finishing rolling is important. As shown in FIGS. 32 and 33, theaustenite grain diameter before finishing rolling is desirably small,and the above values are satisfied when the austenite grain diameter is200 μm or less.

In order to obtain an austenite grain diameter before finishing rollingof 200 μm or less, as shown in FIG. 21, it is necessary to carry out therough rolling in a temperature range of 1000° C. to 1200° C. and carryout rolling once or more at a rolling reduction ratio of at least 20% ormore. However, in order to further enhance homogeneity and enhanceelongation and local deformability, it is desirable to carry out rollingonce or more at a rolling reduction ratio of at least 40% in terms of arough rolling reduction ratio in a temperature range of 1000° C. to1200° C.

In order to obtain austenite grains of 100 μm or less which are morepreferable, one or more times of rolling, a total of two or more timesof rolling at a rolling reduction ratio of 20% or more is furthercarried out. Desirably, rolling is carried out twice or more at 40% ormore. As the rolling reduction ratio and the number of times of rollingincrease, smaller grains can be obtained, but there is a concern thatthe temperature may decrease or scales may be excessively generated whenthe rolling exceeds 70% or the number of times of the rough rollingexceeds 10 times. As such, a decrease in the austenite grain diameterbefore finishing rolling is effective to improve local deformabilitythrough acceleration of recrystallization of austenite during subsequentfinishing rolling, particularly through control of rL or r30.

The reason why refinement of the austenite grain diameter has aninfluence on local deformability is assumed to be that austenite grainboundaries after the rough rolling, that is, austenite grain boundariesbefore the finishing rolling function as one of recrystallization nucleiduring the finishing rolling.

In order to confirm the austenite grain diameter after the roughrolling, it is desirable to cool a sheet piece that is about to befinishing-rolled as rapidly as possible. The sheet piece is cooled at acooling rate of 10° C./s or more, the structure on the cross section ofthe sheet piece is etched, austenite grain boundaries are made toappear, and the austenite grain diameter is measured using an opticalmicroscope. At this time, the austenite grain diameter is measured at amagnification of 50 times or more at 20 sites or more through an imageanalysis or a point counter method. Furthermore, the austenite graindiameter is desirably 100 μm or less in order to enhance localdeformability.

In addition, in order to achieve an average value of the X-ray randomintensity ratio of the {100}<011> to {223}<110> orientation group in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from the steel sheet surface and an X-ray random intensity ratio of the{332}<113> crystal orientation in the above value ranges, based on theT1 temperature determined by the steel sheet components specified in theformula 1 in the finishing rolling after the rough rolling, working iscarried out at a large rolling reduction ratio in a temperature range ofT1+30° C. to T1+200° C., desirably in a temperature range of T1+50° C.to T1+100° C., and working is carried out at a small rolling reductionratio in a temperature range of T1° C. to lower than T1+30° C. Accordingto the above, the local deformability and shape of a final hot-rolledproduct can be secured.

FIGS. 34 to 37 show the relationships between the rolling reductionratios in the respective temperature ranges and the X-ray randomintensity ratios of the respective orientations.

That is, large reduction at a total rolling reduction ratio of 50% ormore in a temperature range of T1+30° C. to T1+200° C. as shown in FIGS.34 and 36 and subsequent light rolling at a total rolling reductionratio of less than 30% or more at T1° C. to lower than T1+30° C. asshown in FIGS. 35 and 37 control the average value of the X-ray randomintensity ratio of the {100}<011> to {22}<110> orientation group in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from the steel sheet surface, and the X-ray random intensity ratio ofthe {332}<113> crystal orientation so as to drastically improve thelocal deformability of the final hot-rolled product. The T1 temperatureis experimentally obtained, and the inventors and the like found fromexperiments that recrystallization in the austenite range of therespective steels is accelerated with the T1 temperature as a basis.

Furthermore, in order to obtain more favorable local deformability, itis important to accumulate strain through the large reduction orrepeatedly recrystallize the structure every rolling. For strainaccumulation, the total rolling reduction ratio needs to be 50% or more,more desirably 60% or more, and still more desirably 70% or more, andthe temperature increase of the steel sheet between passes is desirably18° C. or lower. On the other hand, achieving a rolling reduction ratioof more than 90% is not desirable from the viewpoint of temperaturesecurement or excessive rolling load. Furthermore, in order to enhancethe homogeneity of a hot-rolled sheet, and enhance the localdeformability to the extreme, among the rolling passes in a temperaturerange of T1+30° C. to T1+200° C., in at least one pass, rolling iscarried out at a rolling reduction ratio of 30% or more, and desirablyat 40% or more. Meanwhile, when the rolling reduction ratio exceeds 70%in a pass, there is a concern that the shape may be impaired. In a casein which there is a demand for more favorable workability, it is moredesirable to set the rolling reduction ratio to 30% or more in the final2 passes.

Furthermore, in order to accelerate uniform recrystallization throughreleasing of accumulated strain, it is necessary to suppress as much aspossible the working amount in a temperature range of T1° C. to lowerthan T1+30° C. after the large reduction at T1+30° C. to T1+200° C., andthe total rolling rate at T1° C. to lower than T1+30° C. is set to lessthan 30%. A rolling reduction ratio of 10% or more is desirable from theviewpoint of the sheet shape, but a rolling reduction ratio of 0% isdesirable in a case in which local deformability is focused. When therolling reduction ratio at T1° C. to lower than T1+30° C. exceeds apredetermined range, recrystallized austenite grains are expanded, and,when the retention time is short, recrystallization does notsufficiently proceed, and the local deformability deteriorates. That is,in the manufacturing conditions according to the embodiment, it isimportant to uniformly and finely recrystallize austenite duringfinishing rolling so as to control the texture of a hot-rolled productin order to improve local deformability, such as hole expandingproperties or bending properties.

When rolling is carried out at a lower temperature than the temperaturerange specified above or at a larger rolling reduction ratio than thespecified rolling reduction ratio, the texture of austenite develops,and the X-ray random intensity ratios in the respective crystalorientations, such as the average value of the X-ray random intensityratio of the {100}<011> to {22}<110> orientation group at least in athickness central portion that is in a sheet thickness range of ⅝ to ⅜from a steel sheet surface of less than 4.0, and the X-ray randomintensity ratio of the {332}<113> crystal orientation of 5.0 or less,cannot be obtained in the finally obtained galvanized steel sheet.Meanwhile, when rolling is carried out at a higher temperature than thespecified temperature range or at a smaller rolling reduction ratio thanthe specified rolling reduction ratio, grain coarsening or duplex grainsresults, and, consequently, local deformability significantly degrades.Whether or not the above-specified rolling is carried out can bedetermined from rolling reduction ratio, rolling load, sheet thicknessmeasurement, or the like through actual performance or calculation. Inaddition, since the temperature can be also measured if a thermometer ispresent between stands, and calculation simulation in which working heatgeneration and the like are considered from line speed, rollingreduction ratio, and the like is available, whether or not theabove-specified rolling is carried out can be determined using either orboth of temperature and calculation simulation.

The hot rolling carried out in the above manner ends at a temperature ofAr3 or higher. When the end temperature of the hot rolling is lower thanAr3, since two-phase region rolling in an austenite area and a ferritearea is included, accumulation into the {100}<011> to {223}<110>orientation group becomes strong, and, consequently, local deformabilitysignificantly degrades.

Furthermore, as long as rL and r60 are 0.70 or more and 1.10 or lessrespectively, furthermore, the sheet thickness/minimum bending radius isgreater than or equal to 2.0. In order to achieve the sheetthickness/minimum bending radius of greater than or equal to 2.0, in acase in which a pass in which a rolling reduction ratio is 30% or morein a temperature range of T1+30° C. to T1+200° C. is defined as a largereduction pass, it is important for the waiting time t (seconds) fromcompletion of the final pass of the large reduction pass to initiationof cooling to satisfy the formula 6.

FIGS. 38 and 39 show the relationship among the temperature increase ofthe steel sheet during rolling at T1+30° C. to T1+200° C., the waitingtime t, rL, and r60.

The waiting time t satisfying the formula 6 and, furthermore,suppression of the temperature increase of the steel sheet at T1+30° C.to T1+200° C. to 18° C. or lower in the respective passes are effectiveto obtain uniformly recrystallized austenite.

Further, in a case in which the temperature increase at T1+30° C. toT1+200° C. is too low such that a predetermined rolling reduction ratiocannot be obtained in a range of T1+30° C. to T1+200° C.,recrystallization is suppressed, and, in a case in which the waitingtime t does not satisfy the formula 6, by the time being too long,grains are coarsened and, by the time being too short, recrystallizationdoes not proceed and sufficient local deformability cannot be obtained.

A cooling pattern after hot rolling is not particularly specified, andthe effects of the present invention can be obtained by employing acooling pattern for controlling the structure according to therespective objects. However, when the winding temperature exceeds 680°C., since there is a concern that surface oxidation may proceed orbending properties after cold rolling or annealing may be adverselyinfluenced, the winding temperature is set to a temperature from roomtemperature to 680° C.

During hot rolling, a sheet bar may be joined after rough rolling, andfinishing rolling may be continuously carried out. At this time, a roughbar may be once rolled into a coil shape, stored in a cover having aheat-retention function as necessary, and again rolled back, whereby therough bar is joined. Skin pass rolling may be carried out on thehot-rolled steel sheet as necessary. Skin pass rolling has an effect ofpreventing stretched strain occurring during working forming or flatnesscorrection.

In addition, the steel sheet for which the hot rolling has beencompleted is subjected to pickling, and then cold rolling at a rollingreduction ratio of 20% to 90%. When the rolling reduction ratio is lessthan 20%, there is a concern that sufficient cold-rolled recrystallizedstructures may not be formed, and mixed grains may be formed. Inaddition, when the rolling reduction ratio exceeds 90%, there is aconcern of rupture due to cracking. The effects of the present inventioncan be obtained even when a heat treatment pattern for controlling thestructure in accordance with purposes is employed as the heat treatmentpattern of annealing.

However, in order to obtain a sufficient cold-rolled recrystallizedequiaxed structure and satisfy conditions in the ranges of the presentapplication, it is necessary to heat the steel sheet to a temperaturerange of at least 650° C. to 900° C., anneal the steel sheet for aholding time of 1 second to 300 seconds, and then carry out primarycooling to a temperature range of 720° C. to 580° C. at a cooling rateof 0.1° C./s to 100° C./s. When the holding temperature is lower than650° C., or the holding time is less than 1 second, a sufficientrecovered recrystallized structure cannot be obtained. In addition, whenthe holding temperature exceeds 900° C., or the holding time exceeds 300seconds, there is a concern of oxidation or coarsening of grains. Inaddition, when the cooling rate is less than 0.1° C./s, or thetemperature range exceeds 720° C. in the temporary cooling, there is aconcern that a sufficient amount of transformation may not be obtained.In addition, in a case in which the cooling rate exceeds 100° C./s, orthe temperature range is lower than 580° C., there is a concern ofcoarsening of grains and the like.

After that, according to an ordinary method, a galvanizing treatment iscarried out so as to obtain a galvanized steel sheet.

The structure of the galvanized steel sheet obtained in the embodimentmainly includes ferrite, but may include pearlite, bainite, martensite,austenite, and compounds such as carbonitrides, as metallic structuresother than ferrite. Since the crystal structure of martensite or bainiteis the same as or similar to the crystal structure of ferrite, thestructure may mainly include any of ferrite, bainite, and martensite.

The galvanized steel sheet according to the present invention can beapplied not only to bending working but also to combined formingcomposed mainly of bending, overhanging, drawing, and bending working.

Example 1

The technical content of the hot-rolled steel sheet according to theembodiment will be described using examples of the present invention.

The results of studies in which steels of AA to Bg having the componentcompositions shown in Table 1 were used as examples will be described.

[Table 1]

The steels were cast, reheated as they were or after being cooled toroom temperature, heated to a temperature range of 900° C. to 1300° C.,and then hot-rolled under the conditions of Table 2 or 3, thereby,finally, obtaining 2.3 mm or 3.2 mm-thick hot-rolled steel sheets.

[Table 2]

[Table 3]

Table 1 shows the chemical components of the respective steels, Tables 2and 3 show the respective manufacturing conditions, and Tables 4 and 5show structures and mechanical characteristics.

As an index of local deformability, the hole expanding rate and thelimit bending radius through 90° V-shape bending were used. In bendingtests, C-direction bending and 45°-direction bending were carried out,and the rates were used as the index of the orientation dependency offormability. Tensile tests and the bending tests were based on JIS Z2241and the V block 90° bending tests of JIS Z2248, and hole expanding testswere based on the Japan Iron and Steel Federation standard JFS T1001,respectively. The X-ray random intensity ratio was measured using theEBSD at a 0.5 μm pitch with respect to a ¼ location from the end portionin the width direction in a sheet thickness central portion in a ⅝ to ⅜area of a cross section parallel to the rolling direction. In addition,the r values in the respective directions were measured through theabove methods.

[Table 4]

[Table 5]

Example 2

The technical content of the cold-rolled steel sheet according to theembodiment will be described using examples of the present invention.

The results of studies in which steels of CA to CW having the componentcompositions shown in Table 6 which satisfied the components specifiedin the claims of the present invention and comparative steels of Ca toCg were used as examples will be described.

[Table 6]

The steels were cast, reheated as they were or after being cooled toroom temperature, heated to a temperature range of 900° C. to 1300° C.,then, hot-rolled under the conditions of Table 7, thereby obtaining 2 mmto 5 mm-thick hot-rolled steel sheets. The steel sheets were pickled,cold-rolled into a thickness of 1.2 mm to 2.3 mm, and annealed under theannealing conditions shown in Table 7. After that, 0.5% scan passrolling was carried out, and the steel sheets were provided for materialquality evaluation.

[Table 7]

Table 6 shows the chemical components of the respective steels, andTable 7 shows the respective manufacturing conditions. In addition,Table 8 shows the structures and mechanical characteristics of the steelsheets. As an index of local deformability, the hole expanding rate andthe limit bending radius through V-shape bending were used. In bendingtests, C-direction bending and 45°-direction bending were carried out,and the rates were used as the index of the orientation dependency offormability. Tensile tests and the bending tests were based on JIS Z2241and the V block 90° bending tests of JIS Z2248, and hole expanding testswere based on the Japan Iron and Steel Federation standard JFS T1001,respectively. The X-ray random intensity ratio was measured using theEBSD at a 0.5 μm pitch with respect to a ¼ location from the end portionin the width direction in a sheet thickness central portion in a ⅝ to ⅜area of a cross section parallel to the rolling direction. In addition,the r values in the respective directions were measured through theabove methods.

[Table 8]

Example 3

The technical content of the galvanized steel sheet according to theembodiment will be described using examples of the present invention.

The results of studies in which steels of DA to DL having the componentcompositions shown in Table 9 were used as examples will be described.

[Table 9]

The steels were cast, reheated as they were or after being cooled toroom temperature, heated to a temperature range of 900° C. to 1300° C.,then, cold-rolled under the conditions of Table 10, thereby obtaining 2mm to 5 mm-thick hot-rolled steel sheets. The steel sheets were pickled,cold-rolled into a thickness of 1.2 mm to 2.3 mm, annealed under theannealing conditions shown in Table 10, and continuously subjected toannealing and a galvanized coating or galvanealed coating treatmentusing a galvanized coating bath. After that, 0.5% scan pass rolling wascarried out, and the steel sheets were provided for material qualityevaluation.

[Table 10]

Table 9 shows the chemical components of the respective steels, Table 10shows the respective manufacturing conditions, and Table 11 shows thestructures and mechanical characteristics of the steel sheets under therespective manufacturing conditions.

As an index of local deformability, the hole expanding rate and thelimit bending radius through 90° V-shape bending were used. Tensiletests and the bending tests were based on JIS Z2241 and the V block 90°bending tests of JIS Z 2248, and hole expanding tests were based on theJapan Iron and Steel Federation standard JFS T1001, respectively. TheX-ray random intensity ratio was measured using the EBSD at a 0.5 μmpitch with respect to a ¼ location from the end portion in the widthdirection in a sheet thickness central portion in a ⅜ to ⅝ area of across section parallel to the rolling direction. In addition, the rvalues in the respective directions were measured through the abovemethods.

[Table 11]

As shown in, for example, FIGS. 40, 41, 42, 43, 44, and 45, steel sheetssatisfying the specifications of the present invention had excellenthole expanding properties, bending properties, and small forminganisotropy. Furthermore, steel sheets manufactured in the desirablecondition ranges exhibited superior hole expanding rate and bendingproperties.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, without limitingthe main structure configuration, it is possible to obtain a hot-rolledsteel sheet, a cold-rolled steel sheet, and a galvanized steel sheetwhich are excellent in terms of local deforambility and have a smallorientation influence of formability even when Nb, Ti and the like areadded by controlling the texture in addition to controlling the sizesand shapes of crystal grains.

Therefore, the present invention is highly useful in thesteel-manufacturing industry.

In addition, in the present invention, the strength of the steel sheetis not specified; however, since formability degrades as the strengthincreases as described above, the effects are particularly large in thecase of a high-strength steel sheet, for example, a case in which thetensile strength is 440 MPa or more.

TABLE 1 Chemical components (mass %) (1/4) T1/° C. C Si Mn P S Al N O TiNb AA 851 0.070 0.08 1.30 0.015 0.004 0.040 0.0026 0.0032 — — AB 8650.080 0.31 1.35 0.012 0.005 0.016 0.0032 0.0023 —  0.041 AC 858 0.0600.87 1.20 0.009 0.004 0.038 0.0033 0.0026 —  0.021 AD 865 0.210 0.151.62 0.012 0.003 0.026 0.0033 0.0021  0.021 — AE 861 0.035 0.67 1.880.015 0.003 0.045 0.0028 0.0029 —  0.021 AF 875 0.180 0.48 2.72 0.0090.003 0.050 0.0036 0.0022 — — AG 892 0.060 0.11 2.12 0.010 0.005 0.0330.0028 0.0035  0.036  0.089 AH 903 0.040 0.13 1.33 0.010 0.005 0.0380.0032 0.0026  0.042  0.121 AI 855 0.350 0.52 1.33 0.260 0.003 0.0450.0026 0.0019 — — AJ 1376 0.072 0.15 1.42 0.014 0.004 0.036 0.00220.0025 — 1.5  AK 851 0.110 0.23 1.12 0.021 0.003 0.026 0.0025 0.0023 — —AL 1154 0.250 0.23 1.56 0.024 0.120 0.034 0.0022 0.0023 — — BA 864 0.0780.82 2.05 0.012 0.004 0.032 0.0026 0.0032 0.02 0.02 BB 852 0.085 0.752.25 0.012 0.003 0.035 0.0032 0.0023 — — BC 866 0.110 0.10 1.55 0.02 0.004 0.038 0.0033 0.0026 — 0.04 BD 863 0.350 1.80 2.33 0.012 0.0030.710 0.0033 0.0021 0.02 BE 859 0.120 0.22 1.35 0.015 0.003 0.025 0.00550.0029 — 0.02 BF 884 0.068 0.50 3.20 0.122 0.002 0.040 0.0032 0.00380.03 0.07 BG 858 0.130 0.24 1.54 0.010 0.001 0.038 0.0025 0.0029 — 0.02BH 899 0.035 0.05 2.20 0.010 0.020 0.021 0.0019 0.0023 0.15 0.03 BI 8520.090 1.25 1.88 0.014 0.002 0.030 0.0030 0.0030 — — BJ 852 0.115 1.101.46 0.008 0.002 0.850 0.0034 0.0031 — — BK 861 0.144 0.45 2.52 0.0070.001 0.021 0.0024 0.0031 0.03 — (2/4) B Mg Rem Ca Mo Cr W As V OthersNote AA — — — — — — — — — — Invention steel AB — — — — — — — — — —Invention steel AC — — 0.0015 — — — — — — — Invention steel AD 0.0022 —— — 0.03 0.35 — — — — Invention steel AE — 0.002 — 0.0015 — — — —  0.029— Invention steel AF — 0.002 — — 0.10 — — — 0.10 — Invention steel AG0.0012 — — — — — — — — — Invention steel AH 0.0009 — — — — — — — — —Invention steel AI — — — — — — — — — — Comparative steel AJ — — — — — —— — — — Comparative steel AK — 0.150 — — — — — — — — Comparative steelAL — — — — — 5.0  — — 2.50 — Comparative steel BA — — — — — — — — — —Invention steel BB — — — — — — — — — Co: 0.5% Invention Sn: 0.02% steelBC — — — — — — — — — — Invention steel BD 0.0020 — 0.0035 — — — — — — —Invention steel BE — — — — — — — — — — Invention steel BF — — 0.0044 — —0.10 — — — — Invention steel BG — — — — — — — — — Invention steel BH — —0.0005 0.0009 0.05 — — Invention steel BI — — — — — — — — — — Inventionsteel BJ — — — — — — — — — — Invention steel BK — — — — — — — — — Cu:0.5%, Invention Ni: 0.25%, steel Zr: 0.02% (3/4) T1/° C. C Si Mn P S AlN O Ti Nb BL 853 0.190 1.40 1.78 0.011 0.002 0.018 0.0032 0.0028 — — BM866 0.080 0.10 1.40 0.007 0.002 1.700 0.0033 0.0034 — — BN 852 0.0620.72 2.82 0.009 0.002 0.035 0.0033 0.0022 — — BO 885 0.120 0.80 2.200.008 0.002 0.035 0.0022 0.0035 0.05 — BP 873 0.190 0.55 2.77 0.0090.002 0.032 0.0033 0.0036 0.04 — BQ 852 0.082 0.77 1.82 0.008 0.0030.025 0.0032 0.0031 — — BR 875 0.030 1.00 2.40 0.005 0.001 0.033 0.00220.0011 0.05 0.01 BS 852 0.077 0.45 2.05 0.009 0.003 0.025 0.0029 0.0031— — BT 861 0.142 0.70 2.44 0.008 0.002 0.030 0.0032 0.0035 0.03 — BU 8760.009 0.10 1.40 0.006 0.001 0.003 0.0033 0.0024 0.10 — BV 853 0.150 0.612.20 0.011 0.002 0.028 0.0021 0.0036 — — BW 1043 0.120 0.17 2.26 0.0280.009 0.033 0.0027 0.0019 — — Ba 860 0.440 0.50 2.20 0.008 0.002 0.0350.0021 0.0012 — — Bb 854 0.080 0.45 4.50 0.200 0.002 0.034 0.0041 0.0015— — Bc 914 0.080 0.35 2.00 0.008 0.002 0.033 0.0042 0.0034 0.25 — Bd 9390.070 0.35 2.40 0.008 0.002 0.035 0.0035 0.0026 — 0.25 Be 851 0.090 0.101.00 0.008 0.040 0.036 0.0035 0.0022 — — Bf 952 0.070 0.21 2.20 0.0080.002 0.033 0.0023 0.0036 — — Bg 853 0.140 0.11 1.90 0.008 0.002 0.0320.0044 0.0035 — — (4/4) B Mg Rem Ca Mo Cr W As V Others Note BL 0.0002 —— — — — — — — — Invention steel BM — — — 0.0022 — — — — 0.15 — Inventionsteel BN — — — — — — — — — — Invention steel BO — — — — — — — 0.01 0.20— Invention steel BP — 0.006 — —  0.022 — — — 0.05 — Invention steel BQ0.0002 — — — — — — — — — Invention steel BR — 0.004 0.004 — — 0.80 — — —— Invention steel BS — — — — — — — — — — Invention steel BT 0.0002 — — —— — — — — — Invention steel BU — — — — 0.01 — — — — — Invention steel BV— 0.004 0.005 — — — — — — — Invention steel BW — — — — 0.90 — — — — —Invention steel Ba — — — — — — — — — — Comparative steel Bb — — — — — —— — — — Comparative steel Bc — — — — — — — — — — Comparative steel Bd —— — — — — — — — — Comparative steel Be — — — — — — — — — — Comparativesteel Bf — 0.020 — — — — — — 1.10 — Comparative steel Bg — — 0.15  — — —— — — — Comparative steel

TABLE 2 Manufacturing conditions (1/2) Number of times Rolling reductionTemperature of rolling of rate of 20% Austenite Total rolling increaseduring 20% or more at or more at grain reduction rate at rolling atSteel 1000° C. to 1000° C. to diameter/ T1 + 30° C. to T1 + 30° C. totype T1/° C. 1200° C. 1200° C./% μm T1 + 200° C./% T1 + 200° C./° C. 1AA 851 1 20 150  85 15 2 AA 851 2 45/45 90 95  5 3 AB 865 2 45/45 80 7515 4 AB 865 2 45/45 80 85 18 5 AC 858 2 45/45 95 85 13 6 AC 858 2 45/4595 95 14 7 AD 862 3 40/40/40 75 80 16 8 AE 858 2 45/40 95 80 17 9 AE 8581 50 120  80 18 10 AF 875 3 40/40/40 70 95 18 11 AG 892 3 40/40/40 65 9510 12 AH 903 2 45/45 70 90 13 13 AH 903 2 45/45 95 85 15 14 AF 875 340/40/40 70 65 20 15 AG 892 1 50 120  75 20 16 AG 892 1 50 120  60 21 17AH 903 1 50 120  65 19 18 AH 903 1 50 120  35 12 19 AA 851 2 45/45 90 4520 20 AB 865 2 45/45 80 45 15 21 AV 858 2 40/45 95 75 12 22 AG 892 0 —350  45 30 23 AE 858 1 50 120  80 40 24 AA 851 0 — 250  65 18 25 AC 8580 — 300  85 13 26 AI 855 Cracked during hot rolling 27 AJ 1376 Crackedduring hot rolling 28 AK 851 Cracked during hot rolling 29 AL 1154Cracked during hot rolling (2/2) Total rolling Tf: t: Waiting timereduction Temperature P1: Rolling from completion rate at after finalreduction rate of heavy rolling T1° C. to pass of of final pass pass toSteel lower than heavy rolling of heavy initiation Winding type T1 + 30°C./% pass/° C. rolling pass/% t1 2.5 × t1 of cooling/s t/t1temperature/° C. 1 10 935 40 0.57 1.41 0.8 1.41 600 2  0 892 35 1.744.35 2   1.15 50 3 25 945 37 0.76 1.90 1   1.32 600 4  5 920 31 1.543.86 2.3 1.49 50 5 15 955 31 0.73 1.82 1   1.38 600 6  0 934 40 0.711.78 1   1.41 500 7 25 970 30 0.62 1.56 0.9 1.45 600 8  5 960 30 0.701.75 1   1.42 300 9 15 921 30 1.40 3.50 2   1.43 200 10  0 990 30 0.531.32 0.7 1.32 500 11  0 943 35 1.46 3.65 2.1 1.44 600 12  0 1012 40 0.250.63 0.3 1.19 500 13 10 985 40 0.61 1.52 0.9 1.48 600 14 25 965 34 0.701.75 0.9 1.28 500 15 15 993 30 0.71 1.77 0.8 1.13 500 16 20 945 45 1.062.64 1.1 1.04 600 17 15 967 38 1.05 2.63 1.5 1.43 500 18 45 880 30 3.929.79 5   1.28 100 19 45 930 30 1.08 2.69 5   4.64 600 20 45 1075 30 0.200.50 0.1 0.50 600 21 45 890 30 2.15 5.36 1.3 0.61 600 22 35 910 35 2.446.09 0.5 0.21 400 23 35 860 40 3.02 7.54 9   2.98 600 24 20 850 30 3.137.83 0.3 0.10 800 25 25 890 30 2.15 5.36 2.2 1.03 600 26 Cracked duringhot rolling 27 Cracked during hot rolling 28 Cracked during hot rolling29 Cracked during hot rolling

TABLE 3 Manufacturing conditions (1/2) Number of times Rolling reductionTemperature of rolling of rate of 20% Austenite Total rolling increaseduring 20% or more at or more at grain reduction rate at rolling atSteel 1000° C. to 1000° C. to diameter/ T1 + 30° C. to T1 + 30° C. totype T1/° C. 1200° C. 1200° C./% μm T1 + 200° C./% T1 + 200° C./° C. BA1BA 864 2 45/45 80 85 17 BB1 BB 852 2 45/45 85 80 13 BB2 BB 852 2 45/4580 85 16 BC1 BC 866 2 45/45 80 85 16 BD1 BD 863 1 50 120 85 14 BE2 BE859 2 45/45 80 80 16 BF1 BF 884 2 45/45 75 85 15 BF2 BF 884 1 50 110 8013 BG1 BG 858 3 40/40/40 80 80 15 BH1 BH 899 2 45/45 80 80 12 BI1 BI 8522 45/45 75 90 12 BI2 BI 852 2 45/45 75 80 16 BJ1 BJ 852 3 40/40/40 85 8515 BJ2 BJ 852 2 45/45 75 80 13 BK1 BK 861 3 40/40/40 85 90 13 BK2 BK 8533 40/40/40 85 90 12 BL1 BL 853 2 45/45 80 85 14 BL2 BL 853 2 45/45 80 8017 BM1 BM 866 1 30 140 65 12 BN1 BN 852 2 45/45 75 70 12 BO1 BO 885 245/45 80 60 15 BP1 BP 873 2 45/45 75 85 13 BQ1 BQ 852 2 45/45 80 80 16BR1 BR 875 2 45/45 75 85 12 BS1 BS 852 2 45/45 80 85 12 BS2 BS 852 245/45 75 80 15 BT1 BT 861 2 45/45 80 95 16 BT2 BT 861 2 45/45 85 80 12BU1 BU 876 2 45/45 75 85 12 BV1 BV 853 2 45/45 85 80 11 BW1 BW 1043 1 50120 80 16 Ba1 Ba 860 2 45/45 75 90 16 Bb1 Bb 854 1 50 120 85 12 Bc1 Bc914 2 45/45 75 90 13 Bd1 Bd 939 2 45/45 75 85 12 Be1 Be 851 2 45/45 8065 11 Bf1 Bf 952 2 45/45 80 70 12 Bg1 Bg 853 2 45/45 75 60 12 (2/2)Total rolling Tf: t: Waiting time reduction Temperature P1: Rolling fromcompletion rate at after final reduction rate of heavy rolling T1° C. topass of heavy of final pass pass to Steel lower than rolling of heavyinitiation Winding type T1 + 30° C./% pass/° C. rolling pass/% t1 2.5 ×t1 of cooling/s t/t1 temperature/° C. BA1 0 984 45 0.13 0.33 0.28 2.15500 BB1 0 982 40 0.14 0.34 0.29 2.10 500 BB2 0 922 45 0.66 1.65 1.151.75 500 BC1 0 966 45 0.22 0.55 0.37 1.68 600 BD1 0 963 40 0.34 0.850.49 1.44 600 BE2 0 929 45 0.66 1.65 1.15 1.75 600 BF1 15 944 45 0.892.22 1.04 1.17 500 BF2 0 954 40 0.83 2.08 6.00 7.21 500 BG2 0 958 450.22 0.55 0.37 1.68 600 BH1 20 959 40 1.06 2.65 1.21 1.14 500 BI1 0 95240 0.34 0.85 0.49 1.44 600 BI2 0 922 45 0.66 1.65 1.15 1.75 600 BJ1 0962 45 0.15 0.39 0.30 1.97 600 BJ2 0 922 40 0.83 2.08 1.46 1.75 600 BK10 961 40 0.34 0.85 0.49 1.44 550 BK2 0 923 40 0.83 2.08 0.98 1.18 600BL1 0 953 45 0.22 0.55 0.37 1.68 600 BL2 0 923 50 0.51 1.28 0.66 1.29600 BM1 10 966 40 0.34 0.85 0.49 1.44 500 BN1 0 952 40 0.34 0.85 0.491.44 550 BO1 0 985 45 0.22 0.55 0.37 1.68 600 BP1 0 973 40 0.34 0.850.49 1.44 600 BQ1 0 952 45 0.22 0.55 0.37 1.68 600 BR1 0 985 40 0.240.60 0.39 1.63 500 BS1 0 992 40 0.13 0.33 0.28 2.14 550 BS2 0 922 450.66 1.65 0.81 1.23 550 BT1 15 961 45 0.22 0.55 0.37 1.68 500 BT2 0 93140 0.83 2.08 0.98 1.18 500 BU1 10 976 40 0.34 0.85 0.49 1.44 500 BV1 0953 40 0.34 0.85 0.49 1.44 600 BW1 10 1083 45 1.46 3.66 1.61 1.10 550Ba1 0 960 45 0.22 0.55 0.37 1.68 600 Bb1 0 954 40 0.34 0.85 0.49 1.44600 Bc1 0 994 40 0.64 1.59 0.79 1.24 600 Bd1 0 999 40 1.06 2.65 1.211.14 600 Be1 0 951 40 0.34 0.85 0.49 1.44 600 Bf1 0 1012 40 1.06 2.651.21 1.14 600 Bg1 0 953 40 0.34 0.85 0.49 1.44 600

TABLE 4 The structure and mechanical characteristics of the respectivesteels in the respective manufacturing conditions (1/2) X-ray randomintensity ratio of coarsened {100} <011> to X-ray random grain Steel{223} <110> intensity ratio area type orientation group of {332} <113>rL rC r30 r60 ratio/% 1 2.6 2.2 0.88 0.87 1.04 1.05 5 2 2.2 2.1 0.920.90 0.96 0.98 1 3 2.9 2.8 0.87 0.79 1.05 1.05 5 4 2.7 2.7 0.90 0.851.02 1.03 4 5 3.5 3.2 0.78 0.75 0.98 1.00 6 6 3.0 2.8 0.83 0.85 0.950.98 4 7 5.2 4.1 0.70 0.70 1.08 1.09 7 8 2.9 2.7 0.85 0.90 1.06 1.05 5 93.5 2.9 0.75 0.95 1.02 1.10 5 10 3.0 3.0 0.72 0.75 1.05 1.08 6 11 2.93.0 0.72 0.74 1.07 1.09 6 12 2.9 2.6 0.71 0.72 1.06 1.08 3 13 3.0 2.90.73 0.72 1.10 1.08 5 14 5.4 4.6 0.66 0.73 1.10 1.20 5 15 3.7 3.5 0.650.75 1.05 1.19 4 16 5.4 4.5 0.58 0.70 1.10 1.26 1 17 5.4 3.0 0.64 0.751.02 1.15 5 18 7.2 6.4 0.54 0.67 1.24 1.31 3 19 6.6 5.1 0.69 0.79 1.151.15 29  20 6.9 5.2 0.56 0.65 1.25 1.19 1 21 7.2 5.8 0.65 0.68 1.18 1.151 22 7.6 5.4 0.52 0.65 1.22 1.30 1 23 7.1 6.4 0.63 0.65 1.15 1.23 16  245.4 5.6 0.59 0.75 1.05 1.21 1 25 5.2 5.4 0.68 0.72 1.15 1.10 4 26Cracked during hot rolling 27 Cracked during hot rolling 28 Crackedduring hot rolling 29 Cracked during hot rolling (2/2) 45°-directionSheet bending/ Steel equiaxed Ts × λ/ thickness/minimum C-direction typegrain rate/% TS/MPa El./% λ/% MPa-% bending radius bending ratio Note 174 445 34 145 64525 3.2 1.1 Invention steel 2 80 450 38 180 81000 3.31.0 Invention steel 3 72 605 25 95 57475 3.2 1.2 Invention steel 4 73595 24 115 68425 2.3 1.1 Invention steel 5 75 595 29 85 50575 2.7 1.2Invention steel 6 78 600 28 90 54000 2.3 1.1 Invention steel 7 72 650 1975 48750 2.1 1.5 Invention steel 8 72 625 21 135 84375 3.3 1.1 Inventionsteel 9 72 635 19 118 74930 3.2 1.2 Invention steel 10 78 735 15 7555125 2.5 1.4 Invention steel 11 77 810 19 85 68850 2.3 1.4 Inventionsteel 12 78 790 21 140 110600 2.7 1.4 Invention steel 13 74 795 20 140111300 2.3 1.4 Invention steel 14 69 765 14 60 45900 1.5 1.6 Inventionsteel 15 74 825 18 70 57750 1.6 1.5 Invention steel 16 70 835 17 6554275 1.5 1.8 Invention steel 17 67 830 17 125 103750 1.5 1.5 Inventionsteel 18 59 805 19 60 48300 1.1 2.0 Invention steel 19 29 465 34 8539525 1.2 1.5 Comparative steel 20 70 635 24 65 41275 1.2 1.9Comparative steel 21 79 640 26 45 28800 1.2 1.7 Comparative steel 22 73845 16 45 38025 1.1 2.0 Comparative steel 23 57 670 16 75 50250 1.2 1.8Comparative steel 24 81 405 30 70 28350 1.1 1.6 Comparative steel 25 78650 27 50 32500 1.1 1.5 Comparative steel 26 Cracked during hot rollingComparative steel 27 Cracked during hot rolling Comparative steel 28Cracked during hot rolling Comparative Steel 29 Cracked during hotrolling Comparative steel

TABLE 5 The structure and mechanical characteristics of the respectivesteels in the respective manufacturing conditions (1/4) X-ray randomintensity ratio of coarsened {100} <011> to X-ray random grain Steel{223} <110> intensity ratio area type orientation group of {332} <113>rL rC r30 r60 ratio/% BA1 2.3 2.4 0.83 0.84 0.85 0.88 9 BB1 2.4 2.4 0.840.85 0.86 0.89 9 BB2 2.8 2.8 0.79 0.81 0.90 0.92 6 BC1 2.8 2.9 0.78 0.800.91 0.93 6 BD1 3.5 3.1 0.83 0.84 0.99 0.99 5 BE2 2.8 2.8 0.79 0.81 0.900.92 6 BF1 3.3 3.4 0.72 0.75 0.97 0.98 2 BF2 1.1 1.2 0.95 0.95 0.99 1.0130  BG1 2.8 2.8 0.78 0.80 0.91 0.93 6 BH1 3.4 3.4 0.72 0.76 0.97 0.98 2BI1 3.0 3.2 0.74 0.77 0.94 0.95 5 BI2 2.7 2.8 0.78 0.80 0.90 0.92 6 BJ12.6 2.6 0.82 0.83 0.88 0.91 8 BJ2 2.7 2.8 0.78 0.80 0.90 0.92 7 BK1 3.13.2 0.76 0.79 0.95 0.96 5 BK2 3.4 3.4 0.73 0.76 0.99 0.99 3 BL1 2.8 2.90.78 0.80 0.91 0.93 6 BL2 3.2 3.2 0.74 0.77 0.95 0.96 2 BM1 3.7 2.9 0.870.87 0.99 0.99 5 BN1 3.0 3.0 0.74 0.77 0.92 0.94 5 BO1 2.8 2.6 0.78 0.800.89 0.91 6 BP1 3.0 3.1 0.74 0.77 0.94 0.95 5 (2/4) 45°-direction Sheetbending/ Steel equiaxed Ts × λ/ thickness/minimum C-direction type grainrate/% TS/MPa El./% λ/% MPa-% bending radius bending ratio Note BA1 67785 24 125 98125 6.4 1.0 Invention steel BB1 66 787 24 123 96801 6.3 1.0Invention steel BB2 71 777 24 120 93240 5.0 1.1 Invention steel BC1 72598 28 155 92690 4.8 1.1 Invention steel BD1 74 1216 14  25 30400 4.11.1 Invention steel BE2 69 588 29 158 92904 5.0 1.1 Invention steel BF177 1198 14  65 77870 3.6 1.3 Invention steel BF2 30 1100 5  50 55000 6.01.0 Invention steel BG1 70 594 29 156 92664 4.8 1.1 Invention steel BH175 843 20 101 85143 3.6 1.3 Invention steel BI1 76 593 37 154 91322 4.11.2 Invention steel BI2 69 583 38 160 93280 5.0 1.1 Invention steel BJ169 607 36 157 95299 5.7 1.0 Invention steel BJ2 69 602 36 156 93912 5.01.1 Invention steel BK1 76 1194 16  33 39402 4.1 1.2 Invention steel BK278 1194 16  30 35820 3.5 1.3 Invention steel BL1 72 795 28 116 92220 4.81.1 Invention steel BL2 74 785 28 114 89490 3.9 1.2 Invention steel BM167 592 29 148 87616 4.2 1.1 Invention steel BN1 69 974 17  78 75972 4.31.2 Invention steel BO1 63 874 19 100 87400 5.1 1.1 Invention steel BP174 1483 11  58 86014 4.1 1.2 Invention steel (3/4) X-ray randomintensity ratio of coarsened {100} <011> to X-ray random grain Steel{223} <110> intensity ratio area type orientation group of {332} <113>rL rC r30 r60 ratio/% BQ1 2.8 2.8 0.78 0.80 0.91 0.93 6 BR1 2.8 2.9 0.760.79 0.92 0.93 6 BS1 2.4 2.4 0.83 0.84 0.86 0.89 7 BS2 3.2 3.3 0.72 0.760.96 0.96 2 BT1 2.8 3.0 0.78 0.80 0.92 0.94 5 BT2 3.4 3.3 0.73 0.76 0.980.98 3 BU1 3.0 3.1 0.74 0.77 0.94 0.95 5 BV1 3.1 3.1 0.76 0.79 0.94 0.955 BW1 3.8 3.4 0.78 0.80 1.03 1.03 1 Ba1 2.8 2.9 0.77 0.79 0.96 0.97 6Bb1 6.5 6.1 0.53 0.64 1.27 1.28 5 Bc1 6.2 6.4 0.42 0.56 1.20 1.22 4 Bd16.3 6.4 0.41 0.55 1.19 1.21 3 Be1 3.1 2.8 0.75 0.78 0.91 0.93 5 Bf1 6.46.3 0.42 0.56 1.18 1.20 3 Bg1 3.0 2.3 0.74 0.77 0.90 0.92 5 (4/4)45°-direction Sheet bending/ Steel equiaxed Ts × λ/ thickness/minimumC-direction type grain rate/% TS/MPa El./% λ/% MPa-% bending radiusbending ratio Note BQ1 70 599 32 155  92845 4.8 1.1 Invention steel BR172 1110 15 70 77700 4.6 1.1 Invention steel BS1 67 594 32 163  96822 6.31.0 Invention steel BS2 74 590 32 152  89680 3.7 1.2 Invention steel BT175 1004 19 74 74296 4.6 1.1 Invention steel BT2 75 989 19 71 70219 3.61.2 Invention steel BU1 74 665 26 140  93100 4.1 1.2 Invention steel BV172 755 22 121  91355 4.2 1.2 Invention steel BW1 76 1459 12 51 74409 3.41.2 Invention steel Ba1 73 892 14 21 18732 4.5 1.2 Comparative steel Bb134 912 12 27 24624 1.2 2.1 Comparative steel Bc1 38 892 15 61 54412 1.02.4 Comparative steel Bd1 27 1057 8 18 19026 1.0 2.4 Comparative steelBe1 67 583 26 83 48389 4.5 1.1 Comparative steel Bf1 72 1079 13 14 151061.0 2.3 Comparative steel Bg1 66 688 21 72 49536 5.0 1.1 Comparativesteel

TABLE 6 Chemical components (mass %) (1/2) T1/° C. C Si Mn P S Al N O TiNb CA 864 0.078 0.82 2.05 0.012 0.004 0.032 0.0026 0.0032 0.02 0.02 CB852 0.085 0.75 2.25 0.012 0.003 0.035 0.0032 0.0023 — — CC 866 0.1100.10 1.55 0.020 0.004 0.038 0.0033 0.0026 — 0.04 CD 863 0.350 1.80 2.330.012 0.003 0.710 0.0033 0.0021 0.02 — CE 859 0.120 0.22 1.35 0.0150.003 0.025 0.0055 0.0029 — 0.02 CF 884 0.068 0.50 3.20 0.122 0.0020.040 0.0032 0.0038 0.03 0.07 CG 858 0.130 0.24 1.54 0.010 0.001 0.0380.0025 0.0029 — 0.02 CH 899 0.035 0.05 2.20 0.010 0.020 0.021 0.00190.0023 0.15 0.03 CI 852 0.090 1.25 1.88 0.014 0.002 0.030 0.0030 0.0030— — CJ 852 0.115 1.10 1.46 0.008 0.002 0.850 0.0034 0.0031 — — CK 8610.144 0.45 2.52 0.007 0.001 0.021 0.0024 0.0031 0.03 — CL 853 0.190 1.401.78 0.011 0.002 0.018 0.0032 0.0028 — — CM 866 0.080 0.10 1.40 0.0070.002 1.700 0.0033 0.0034 — — CN 852 0.062 0.72 2.82 0.009 0.002 0.0350.0033 0.0022 — — CO 885 0.120 0.80 2.20 0.008 0.002 0.035 0.0022 0.00350.05 — CP 873 0.190 0.55 2.77 0.009 0.002 0.032 0.0033 0.0036 0.04 — CQ852 0.082 0.77 1.82 0.008 0.003 0.025 0.0032 0.0031 — — CR 875 0.0301.00 2.40 0.005 0.001 0.033 0.0022 0.0011 0.05 0.01 CS 852 0.077 0.452.05 0.009 0.003 0.025 0.0029 0.0031 — — CT 861 0.142 0.70 2.44 0.0080.002 0.030 0.0032 0.0035 0.03 — CU 876 0.009 0.10 1.40 0.006 0.0010.003 0.0033 0.0024 0.10 — CV 853 0.150 0.61 2.20 0.011 0.002 0.0280.0021 0.0036 — — CW 1043 0.120 0.17 2.26 0.028 0.009 0.033 0.00270.0019 — — Ca 860 0.440 0.50 2.20 0.008 0.002 0.035 0.0021 0.0012 — — Cb854 0.080 0.45 4.50 0.200 0.002 0.034 0.0041 0.0015 — — Cc 914 0.0800.35 2.00 0.008 0.002 0.033 0.0042 0.0034 0.25 — Cd 939 0.070 0.35 2.400.008 0.002 0.035 0.0035 0.0026 — 0.25 Ce 851 0.090 0.10 1.00 0.0080.040 0.036 0.0035 0.0022 — — Cf 952 0.070 0.21 2.20 0.008 0.002 0.0330.0023 0.0036 — — Cg 853 0.140 0.11 1.90 0.008 0.002 0.032 0.0044 0.0035— — (2/2) B Mg Rem Ca Mo Cr W As V Others Note CA — — — — — — — — — —Invention steel CB — — — — — — — — — Co: 0.5%, Invention Sn: 0.02 steelCC — — — — — — — — — — Invention steel CD 0.0020 — 0.0035 — — — — — — —Invention steel CE — — — — — — — — — — Invention steel CF — — 0.0044 — —0.1 — — — — Invention steel CG — — — — — — — — — — Invention steel CH —— 0.0005 0.0009 — — 0.05 — — — Invention steel CI — — — — — — — — — —Invention steel CJ — — — — — — — — — — Invention steel CK — — — — — — —— — Cu: 0.5%, Invention Ni: 0.25 steel Zr: 0.02% CL 0.0002 — — — — — — —— — Invention steel CM — — — 0.0022 — — — — 0.15 — Invention steel CN —— — — — — — — — — Invention steel CO — — — — — — — 0.01 0.20 — Inventionsteel CP — 0.0055 — — 0.022 — — — 0.05 — Invention steel CQ 0.0002 — — —— — — — — — Invention steel CR — 0.0040 0.004  — — 0.8 — — — — Inventionsteel CS — — — — — — — — — — Invention steel CT 0.0002 — — — — — — — — —Invention steel CU — — — — 0.010 — — — — — Invention steel CV — 0.00400.005  — — — — — — — Invention steel CW — — — — 0.90  — — — — —Invention steel Ca — — — — — — — — — — Comparative steel Cb — — — — — —— — — — Comparative steel Cc — — — — — — — — — — Comparative steel Cd —— — — — — — — — — Comparative steel Ce — — — — — — — — — — Comparativesteel Cf — 0.020  — — — — — — 1.10 — Comparative steel Cg — — 0.15  — —— — — — — Comparative steel

TABLE 7 Manufacturing conditions (1/2) P1: Rolling Number of RollingTotal Total Tf: reduction times of reduction rolling Temperature rollingTemperature rate rolling of rate of reduction increase reduction afterfinal of final 20% or 20% or Austenite rate at during rate at pass ofpass more at more at grain T1 + 30° C. rolling at T1° C. to heavy ofheavy Steel 1000° C. to 1000° C. to diameter/ to T1 + 30° C. to lowerthan rolling rolling type T1/° C. 1200° C. 1200° C./% μm T1 + 200° C./%T1 + 200° C./° C. T1 + 30° C./% pass/° C. pass/% CA1 CA 864 2 45/45 8085 16 0 984 45 CA2 CA 864 2 45/45 85 80 15 10  934 45 CB1 CB 852 2 45/4585 80 12 0 982 40 CB2 CB 852 2 45/45 80 85 15 0 922 45 CC1 CC 866 245/45 80 85 15 0 966 45 CC2 CC 866 0 — 250  80 16 0 936 45 CD1 CD 863 150 120  85 12 0 963 40 CD2 CD 863 2 50 130  35 19 0 963 35 CE1 CE 859 245/45 90 95 12 40  909 40 CE2 CE 859 2 45/45 80 80 17 0 929 45 CF1 CF884 2 45/45 75 85 15 15  944 45 CF2 CF 884 1 50 110  80 11 0 954 40 CG1CG 858 3 40/40/40 80 80 15 0 958 45 CG2 CG 858 2 40/40/40 80 80 12 10 928 40 CH1 CH 899 2 45/45 80 80 12 20  959 40 CI1 CI 852 2 45/45 75 9014 0 952 40 CI2 CI 852 2 45/45 75 80 15 0 922 45 CJ1 CJ 852 3 40/40/4085 85 11 0 962 45 CJ2 CJ 852 2 45/45 75 80 12 0 922 40 CK1 CK 861 340/40/40 85 90 12 0 961 40 CK2 CK 853 3 40/40/40 85 90 14 0 923 40 CL1CL 853 2 45/45 80 85 17 0 953 45 CL2 CL 853 2 45/45 80 80 13 0 923 50CM1 CM 866 1 20 150  65 17 10  966 40 CM2 CM 866 1 50 150  60 11 0 96650 CN1 CN 852 2 45/45 75 70 15 0 952 40 CO1 CO 885 2 45/45 80 60 14 0985 45 CO2 CO 885 1 50 120  20 15 10  1100 45 CP1 CP 873 2 45/45 75 8512 0 973 40 CQ1 CQ 852 2 45/45 80 80 16 0 952 45 CR1 CR 875 2 45/45 7585 11 0 985 40 CS1 CS 852 2 45/45 80 85 12 0 992 40 CS2 CS 852 2 45/4575 80 15 0 922 45 CT1 CT 861 2 45/45 80 95 14 15  961 45 CT2 CT 861 245/45 85 80 13 0 931 40 CU1 CU 876 2 45/45 75 85 13 10  976 40 CV1 CV853 2 45/45 85 80 12 0 953 40 CW1 CW 1043 1 50 130  80 16 10  1083 45Ca1 Ca 860 2 45/45 75 90 15 0 960 45 Cb1 Cb 854 1 50 120  85 12 0 954 40Cc1 Cc 914 2 45/45 75 90 12 0 994 40 Cd1 Cd 939 2 45/45 75 85 13 0 99940 Ce1 Ce 851 2 45/45 80 65 11 0 951 40 Cf1 Cf 952 2 45/45 80 70 13 01012 40 Cg1 Cg 853 2 45/45 75 60 12 0 953 40 (2/2) t: Waiting time fromcompletion of heavy Cold rolling pass Winding rolling AnnealingAnnealing Primary Primary Steel to initiation temperature/ reductiontemperature/ holding cooling cooling stop type t1 2.5 × t1 of cooling/st/t1 ° C. rate/% ° C. time/s rate/° C./s temperature/° C. CA1 0.13 0.330.28 2.15 500 45 790 60 30 280 CA2 0.66 1.65 1.15 1.75 500 45 660 60 30280 CB1 0.14 0.34 0.29 2.10 500 45 850 30 30 270 CB2 0.66 1.65 1.15 1.75500 45 850 90 100 270 CC1 0.22 0.55 0.37 1.68 600 50 800 30 120 350 CC20.66 1.65 1.15 1.75 600 50 800 30 120 350 CD1 0.34 0.85 0.49 1.44 600 40820 40 100 290 CD2 0.51 1.28 0.70 1.37 600 40 820 40 30 290 CE1 1.323.30 1.47 1.11 600 50 740 40 120 350 CE2 0.66 1.65 1.15 1.75 600 50 74040 30 350 CF1 0.89 2.22 1.04 1.17 500 40 830 90 100 300 CF2 0.83 2.086.00 7.21 500 40 830 90 100 300 CG1 0.22 0.55 0.37 1.68 600 55 760 30 30330 CG2 0.83 2.08 0.04 0.05 600 40 760 30 100 330 CH1 1.06 2.65 1.211.14 500 45 850 90 120 320 CI1 0.34 0.85 0.49 1.44 600 50 780 30 100 400CI2 0.66 1.65 1.15 1.75 600 50 780 90 30 400 CJ1 0.15 0.39 0.30 1.97 60050 780 30 30 410 CJ2 0.83 2.08 1.46 1.75 600 50 780 90 100 410 CK1 0.340.85 0.49 1.44 550 40 855 30 30 270 CK2 0.83 2.08 0.98 1.18 600 45 80090 30 400 CL1 0.22 0.55 0.37 1.68 600 45 800 30 30 400 CL2 0.51 1.280.66 1.29 600 45 800 30 100 400 CM1 0.34 0.85 0.49 1.44 500 50 840 60100 300 CM2 0.15 0.38 0.25 1.67 500 20 840 60 100 300 CN1 0.34 0.85 0.491.44 550 40 870 30 120 325 CO1 0.22 0.55 0.37 1.68 600 40 800 30 100 270CO2 0.66 1.65 1.15 1.75 600 40 800 30 100 270 CP1 0.34 0.85 0.49 1.44600 40 800 40 30 250 CQ1 0.22 0.55 0.37 1.68 600 50 810 40 110 350 CR10.24 0.60 0.39 1.63 500 40 830 90 100 350 CS1 0.13 0.33 0.28 2.14 550 55780 60 30 320 CS2 0.66 1.65 0.81 1.23 550 45 780 60 100 320 CT1 0.220.55 0.37 1.68 500 50 870 30 100 350 CT2 0.83 2.08 0.98 1.18 500 50 87030 30 350 CU1 0.34 0.85 0.49 1.44 500 45 850 30 120 350 CV1 0.34 0.850.49 1.44 600 50 860 40 100 320 CW1 1.46 3.66 1.61 1.10 550 40 800 40120 350 Ca1 0.22 0.55 0.37 1.68 600 45 820 30 100 350 Cb1 0.34 0.85 0.491.44 600 45 820 30 100 350 Cc1 0.64 1.59 0.79 1.24 600 45 820 30 100 350Cd1 1.06 2.65 1.21 1.14 600 45 820 30 100 350 Ce1 0.34 0.85 0.49 1.44600 50 820 30 100 350 Cf1 1.06 2.65 1.21 1.14 600 40 820 30 100 350 Cg10.34 0.85 0.49 1.44 600 55 820 30 100 350

TABLE 8 The structure and mechanical characteristics of the respectivesteels in the respective manufacturing conditions (1/4) X-ray randomintensity ratio of coarsened {100} <011> to X-ray random grain Steel{223} <110> intensity ratio area type orientation group of {332} <113>rL rC r30 r60 ratio/% CA1 CA 2.6 2.5 0.83 0.84 0.85 0.88 9 CA2 CA 4.43.0 0.80 0.81 0.90 0.92 15  CB1 CB 2.1 2.6 0.84 0.85 0.86 0.89 8 CB2 CB2.5 3.0 0.79 0.81 0.90 0.92 6 CC1 CC 3.0 2.5 0.78 0.80 0.91 0.93 5 CC2CC 5.0 3.5 0.40 0.40 1.26 1.15 15  CD1 CD 3.1 3.8 0.83 0.84 0.99 0.99 5CD2 CD 5.1 5.8 0.84 0.85 0.95 0.96 12  CE1 CE 5.2 7.1 0.73 0.75 1.011.01 8 CE2 CE 3.6 2.5 0.79 0.81 0.90 0.92 5 CF1 CF 3.2 4.0 0.72 0.750.97 0.98 3 CF2 CF 1.1 1.2 0.95 0.95 0.99 1.01 30  CG1 CG 3.4 2.0 0.780.80 0.91 0.93 4 CG2 CG 5.1 5.2 0.61 0.66 1.40 1.38 30  CH1 CH 3.1 3.60.72 0.76 0.97 0.98 1 CI1 CI 3.5 2.8 0.74 0.77 0.94 0.95 3 CI2 CI 3.22.5 0.78 0.80 0.90 0.92 5 CJ1 CJ 2.9 2.2 0.82 0.83 0.88 0.91 7 CJ2 CJ3.2 2.5 0.78 0.80 0.90 0.92 5 CK1 CK 2.7 3.8 0.76 0.79 0.95 0.96 5 CK2CK 3.5 3.5 0.73 0.76 0.99 0.99 2 CL1 CL 3.0 3.0 0.78 0.80 0.91 0.93 6CL2 CL 3.4 3.4 0.74 0.77 0.95 0.96 3 (2/4) Sheet 45°-direction equiaxedthickness/minimum bending/ Steel grain bending radius C-direction typerate/% TS/MPa El./% λ/% (C bending) bending ratio Note CA1 67 785 24 1215.8 1.0 Invention steel CA2 29 805 15  61 0.6 1.6 Comparative steel CB166 788 24 130 6.5 1.0 Invention steel CB2 71 778 24 125 5.1 1.1Invention steel CC1 72 598 28 154 4.9 1.1 Invention steel CC2 39 598 22 81 1.2 2.9 Comparative steel CD1 74 1216 14  29 3.9 1.1 Invention steelCD2 58 1211 8  10 0.4 1.7 Comparative steel CE1 81 585 29  82 0.8 1.8Comparative steel CE2 69 588 29 151 4.6 1.1 Invention steel CF1 77 119814  66 3.3 1.3 Invention steel CF2 30 1100 5  50 6.0 1.0 Invention steelCG1 70 594 29 150 5.0 1.1 Invention steel CG2 30 544 26  71 1.4 2.1Comparative steel CH1 75 844 20 104 3.6 1.3 Invention steel CI1 76 59337 150 4.1 1.2 Invention steel CI2 69 583 38 155 4.9 1.1 Invention steelCJ1 69 608 36 153 5.7 1.0 Invention steel CJ2 69 603 36 151 4.9 1.1Invention steel CK1 76 1194 16  38 3.9 1.2 Invention steel CK2 78 119416  30 3.4 1.3 Invention steel CL1 72 795 28 114 4.5 1.1 Invention steelCL2 74 785 28 112 3.6 1.2 Invention steel (3/4) intensity ratio of {100}<011> to grain Steel {223} <110> intensity ratio area type orientationgroup of {332} <113> ratio/% CM1 CM 2.9 2.8 0.89 0.89 1.00 1.00 3 CM2 CM2.6 5.5 0.93 0.92 0.96 0.97 15  CN1 CN 2.6 3.8 0.74 0.77 0.92 0.94 5 CO1CO 3.0 3.5 0.78 0.80 0.89 0.91 7 CO2 CO 5.0 5.5 0.58 0.58 1.18 1.31 17 CP1 CP 3.3 3.8 0.74 0.77 0.94 0.95 5 CQ1 CQ 2.9 2.5 0.78 0.80 0.91 0.935 CR1 CR 2.8 3.6 0.76 0.79 0.92 0.93 6 CS1 CS 2.8 2.6 0.83 0.84 0.860.89 7 CS2 CS 3.7 3.5 0.72 0.76 0.96 0.96 2 CT1 CT 2.3 2.5 0.78 0.800.92 0.94 4 CT2 CT 2.8 3.0 0.73 0.76 0.98 0.98 1 CU1 CU 2.8 3.3 0.740.77 0.94 0.95 4 CV1 CV 2.7 2.8 0.76 0.79 0.94 0.95 3 CW1 CW 3.6 4.10.79 0.81 1.05 1.04 2 Ca1 Ca 2.8 3.0 0.77 0.79 0.96 0.97 6 Cb1 Cb 8.19.3 0.53 0.64 1.27 1.28 4 Cc1 Cc 8.3 9.5 0.42 0.56 1.20 1.22 3 Cd1 Cd8.4 9.6 0.41 0.55 1.19 1.21 2 Ce1 Ce 3.1 2.8 0.75 0.78 0.91 0.93 3 Cf1Cf 6.4 8.1 0.42 0.56 1.18 1.20 3 Cg1 Cg 3.1 2.3 0.74 0.77 0.90 0.92 2(4/4) Sheet 45°-direction equiaxed thickness/minimum bending/ Steelgrain bending radius C-direction type rate/% TS/MPa El./% λ/% (Cbending) bending ratio Note CM1 67 592 29 157  5.0 1.1 Invention steelCM2 30 592 25 99 0.5 1.5 Comparative steel CN1 69 974 17 84 4.1 1.2Invention steel CO1 63 874 19 98 4.2 1.1 Invention steel CO2 29 884 1423 1.4 2.0 Invention steel CP1 74 1483 11 56 3.6 1.2 Invention steel CQ170 600 32 154  5.0 1.1 Invention steel CR1 72 1110 15 71 4.2 1.1Invention steel CS1 67 594 32 157  5.7 1.0 Invention steel CS2 74 590 32149  3.4 1.2 Invention steel CT1 75 1004 19 82 5.5 1.1 Invention steelCT2 75 989 19 78 4.1 1.2 Invention steel CU1 74 665 26 143  4.2 1.2Invention steel CV1 72 756 22 126  4.8 1.2 Invention steel CW1 76 145912 53 3.1 1.2 Invention steel Ca1 73 893 14 21 4.4 1.2 Comparative steelCb1 34 912 12 28 0.8 2.1 Comparative steel Cc1 38 893 15 61 0.7 2.4Comparative steel Cd1 27 1058 8 18 0.7 2.4 Comparative steel Ce1 67 58326 83 4.5 1.1 Comparative steel Cf1 72 1079 13 14 0.9 2.3 Comparativesteel Cg1 66 688 21 72 5.0 1.1 Comparative steel

TABLE 9 Chemical components (mass %) T1/° C. C Si Mn P S Al N O Ti Nb BDA 857 0.114 0.05 2.15 0.012 0.004 0.590 0.0026 0.0032 — — 0.0005 DB 8680.087 0.62 2.03 0.012 0.003 0.180 0.0032 0.0023 0.022 0.017 0.0012 DC852 0.140 0.87 1.20 0.009 0.004 0.038 0.0033 0.0026 — — — DD 858 0.1450.10 2.33 0.012 0.003 0.710 0.0033 0.0021 0.017 — 0.0005 DE 873 0.2200.13 2.96 0.015 0.003 0.120 0.0029 0.0029 0.024 0.021 — DF 882 0.0680.50 2.31 0.009 0.002 0.040 0.0032 0.0038 0.03  0.065 — DG 851 0.0610.11 2.20 0.010 0.001 0.038 0.0025 0.0029 — — — DH 900 0.035 0.05 1.800.010 0.001 0.021 0.0019 0.0023 0.17  0.02  0.0014 DI 861 0.410 0.082.60 0.190 0.002 0.041 0.0029 0.003 — — — DJ 1220 0.051 0.07 1.67 0.0080.002 0.029 0.0034 0.0031 0.65  0.59  — DK 853 0.150 0.61 2.20 0.0110.002 0.028 0.0021 0.0036 — — — DL 1045 0.120 0.17 2.26 0.028 0.0900.033 0.0027 0.0019 — — 0.0520 Mg Rem Ca Mo Cr V W As Others DA — — — 0.04 — — — — — DB — — — — 0.44 — — — — DC — — — — — — — — — DD — 0.0014— — — — — — — DE 0.0035 — 0.0015 — — 0.029 — — — DF — 0.0021 — — — — — —— DG — — — — — — 0.05 0.01 Cu: 0.5%, Ni: 0.25%, Co: 0.5, Sn: 0.02%, Zr:0.02% DH — 0.0005 0.0009 — — — — — DI — — — — — — — — — DJ — — — — — — —— — DK 0.090  0.10  — — — — — — — DL — — — 1.9 — — — — —

TABLE 10 Manufacturing conditions (1/2) P1: Rolling Number of RollingTotal Total reduction times of reduction rolling Temperature rolling Tf:rate rolling of rate of reduction increase reduction Temperature offinal 20% or 20% or Austenite rate at during rate at after final passmore at more at grain T1 + 30° C. rolling at T1° C. to pass of of heavySteel 1000° C. to 1000° C. to diameter/ to T1 + 30° C. to lower thanheavy rolling rolling type T1/° C. 1200° C. 1200° C./% μm T1 + 200° C./%T1 + 200° C./° C. T1 + 30° C./% pass/° C. pass 30 DA 857 1 50 130  90 15 0 955 45 31 DA 857 2 45/45 85 85 10  0 975 40 32 DB 868 2 45/45 85 8010 10 950 35 33 DB 868 2 45/45 90 85 10  5 925 35 34 DC 852 2 45/45 9085 15 15 960 30 35 DC 852 2 45/45 95 95 17  0 935 35 36 DD 858 340/40/40 70 85 15 25 980 30 37 DE 873 2 45/45 85 80 17  5 955 30 38 DE873 1 50 110  80 18 15 925 30 39 DF 882 3 40/40/40 75 90 18  0 965 35 40DG 851 3 40/40/40 95 85 10  0 945 35 41 DH 900 2 45/45 75 90 13  0 99040 42 DH 900 2 45/45 80 85 15 10 985 40 43 DF 882 1 50 100  65 20 25 93545 44 DG 851 1 50 150  70 20 15 905 45 45 DG 851 1 20 150  60 21 20 89045 46 DH 900 1 50 120  65 19 10 950 45 47 DH 900 1 50 120  35 12 45 88030 48 DA 857 2 45/45 90 45 20 45 900 30 49 DB 868 2 45/45 90 45 15 451050 30 50 DC 852 2 40/45 85 70 15 45 890 30 51 DG 851 0 — 370  45 30 35885 45 52 DE 873 1 50 120  80 40 35 860 40 53 DA 857 0 — 240  60 18 20855 30 54 DC 852 0 — 220  85 14 25 880 45 55 DA 852 2 45/45 85 85 10  0975 40 56 DB 852 2 45/45 90 85 10  5 925 35 57 DC 852 2 45/45 90 85 2515 910 45 58 DG 851 3 40/40/40 95 85 22  0 905 40 59 DI 861 Crackedduring casting or hot rolling 60 DJ 1220 Cracked during casting or hotrolling 61 DK 853 Cracked during casting or hot rolling 62 DL 1045Cracked during casting or hot rolling (2/2) t: Waiting time fromcompletion of heavy Cold rolling pass rolling Annealing Primary PrimarySteel to initiation Winding reduction Annealing holding cooling coolingstop type t1 2.5 × t1 of cooling/s t/t1 temperature/° C. rate/%temperature/° C. time/s rate/° C./s temperature/° C. 30 0.23 0.58 0.301.28 580 60 820 60 3 650 31 0.18 0.45 0.20 1.11 520 60 820 60 3 650 320.79 1.98 1.10 1.39 550 50 840 30 5 680 33 1.32 3.29 1.90 1.44 600 50840 30 5 680 34 0.61 1.54 0.90 1.46 550 50 830 40 3 640 35 0.77 1.931.00 1.29 570 50 830 40 3 640 36 0.45 1.12 0.60 1.34 530 45 850 90 2 70037 1.02 2.55 1.50 1.47 600 40 825 90 2 680 38 1.64 4.10 2.40 1.46 600 40825 90 2 680 39 0.78 1.94 1.00 1.29 620 60 850 30 5 650 40 0.60 1.510.90 1.49 600 60 860 30 5 650 41 0.48 1.19 0.70 1.47 450 50 680 30 5 62042 0.55 1.38 0.70 1.26 450 50 680 30 5 620 43 1.07 2.67 2.00 1.88 620 60850 30 5 650 44 1.05 2.63 1.50 1.43 600 60 860 30 5 650 45 1.51 3.772.60 1.72 600 60 860 30 5 650 46 1.16 2.90 1.50 1.29 600 60 860 30 5 65047 3.80 9.49 4.00 1.05 600 60 860 30 5 650 48 1.85 4.62 4.80 2.60 580 60820 60 3 650 49 0.13 0.32 0.10 0.77 550 50 840 30 5 680 50 1.98 4.951.00 0.51 550 50 840 30 5 680 51 1.68 4.20 0.40 0.24 600 40 825 90 2 68052 3.69 9.22 9.00 2.44 530 45 850 90 2 700 53 3.15 7.88 0.80 0.25 580 60820 60 3 650 54 1.87 4.69 2.00 1.07 570 50 830 40 3 640 55 0.16 0.390.20 1.28 720 60 780 60   0.05 725 56 0.96 2.41 2.00 2.08 600 50 950  0.5 5 600 57 0.93 2.32 1.00 1.08 750 10 830 40 3 640 58 1.22 3.06 1.301.06 600 60 600 30 5 650 59 Cracked during casting or hot rolling 60Cracked during casting or hot rolling 61 Cracked during casting or hotrolling 62 Cracked during casting or hot rolling

TABLE 11 The structure and mechanical characteristics of the respectivesteels in the respective manufacturing conditions (1/2) X-ray randomintensity ratio of {100} <011> to X-ray random Steel {223} <110>intensity ratio type orientation group of {332} <113> rL rC r30 r60 30DA 2.5 2.2 0.81 0.86 0.97 0.98 31 DA 2.4 2.3 0.85 0.82 0.92 0.91 32 DB2.1 2.3 0.90 0.93 0.92 0.98 33 DB 2.3 2.5 0.88 0.91 0.98 1.00 34 DC 2.52.3 0.78 0.75 0.85 0.82 35 DC 2.6 2.8 0.85 0.89 0.98 1.00 36 DD 3.0 3.10.70 0.70 1.08 1.08 37 DE 2.9 3.0 0.76 0.80 1.06 1.05 38 DE 3.3 3.0 0.721.00 0.97 1.09 39 DF 2.3 2.4 0.85 0.88 1.03 1.05 40 DG 2.4 2.3 0.82 0.901.00 0.98 41 DH 2.7 2.8 0.73 0.75 0.98 1.00 42 DH 2.9 3.0 0.75 0.78 0.951.10 43 DF 3.9 4.8 0.63 0.76 1.05 1.20 44 DG 3.4 3.7 0.62 0.77 1.08 1.1945 DG 3.9 4.8 0.60 0.75 1.10 1.28 46 DH 3.9 4.9 0.62 0.80 1.04 1.17 47DH 6.7 6.7 0.51 0.61 1.25 1.30 48 DA 4.1 5.3 0.63 0.68 1.12 1.20 49 DB5.8 5.2 0.55 0.69 1.18 1.26 50 DC 6.8 5.9 0.60 0.65 1.13 1.15 51 DG 7.25.1 0.50 0.69 1.20 1.29 52 DE 6.8 6.0 0.50 0.65 1.16 1.20 53 DA 3.9 5.20.59 0.75 1.06 1.24 54 DC 3.8 5.1 0.68 0.72 1.18 1.10 55 DA 4.2 5.1 0.670.65 1.15 1.16 56 DB 5.8 5.2 0.69 0.60 1.11 1.13 57 DC 4.9 5.8 0.54 0.650.90 1.11 58 DG 6.5 6.1 0.52 0.60 0.89 1.13 59 DI Cracked during castingor hot rolling 60 DJ Cracked during casting or hot rolling 61 DK Crackedduring casting or hot rolling 62 DL Cracked during casting or hotrolling (2/2) Sheet Steel TS × λ/ thickness/minimum type TS/MPa El./%λ/% MPa-% bending radius Note 30 1000 16 55 55000 3.6 Invention steel 311010 17 60 60600 4.0 Invention steel 32 1050 16 65 68250 5.3 Inventionsteel 33 1065 15 70 74550 5.3 Invention steel 34 1230 13 60 73800 3.6Invention steel 35 1250 12 55 68750 4.5 Invention steel 36 1275 10 5063750 3.2 Invention steel 37 1485 9 50 74250 2.6 Invention steel 38 14758 55 81125 2.3 Invention steel 39 805 24 75 60375 2.8 Invention steel 40635 32 60 38100 4.7 Invention steel 41 785 22 145 113825 3.6 Inventionsteel 42 800 21 140 112000 3.0 Invention steel 43 840 19 60 50400 1.8Invention steel 44 640 30 50 32000 1.8 Invention steel 45 630 31 4528350 1.6 Invention steel 46 825 17 100 82500 1.6 Invention steel 47 80519 80 64400 0.9 Comparative steel 48 980 18 30 29400 0.9 Comparativesteel 49 1100 12 45 49500 0.8 Comparative steel 50 990 16 35 34650 0.9Comparative steel 51 650 29 40 26000 0.9 Comparative steel 52 1490 8 3044700 0.7 Comparative steel 53 985 16 35 34475 1.1 Comparative steel 541265 9 45 56925 1.1 Comparative steel 55 890 17 30 26700 0.8 Comparativesteel 56 1150 10 35 40250 0.8 Comparative steel 57 1240 12 35 43400 0.9Comparative steel 58 560 30 40 22400 0.9 Comparative steel 59 Crackedduring casting or hot rolling Comparative steel 60 Cracked duringcasting or hot rolling Comparative steel 61 Cracked during casting orhot rolling Comparative steel 62 Cracked during casting or hot rollingComparative steel

1. A method of manufacturing the hot-rolled steel sheet, the methodcomprising, first hot rolling carried out at least once at a rollingreduction ratio of 20% or more in a temperature range of 1000° C. to1200° C., and an austenite grain diameter is set to 200 μm or less,wherein an ingot or slab containing, by mass %: C: 0.0001% to 0.40%, Si:0.001% to 2.5%, Mn: 0.001% to 4.0%, P: 0.001% to 0.15%, S: 0.0005% to0.03%, Al: 0.001% to 2.0%, N: 0.0005% to 0.01%, O: 0.0005% to 0.01%, andfurther comprising one or two or more of: Ti: 0.001% to 0.20%, Nb:0.001% to 0.20%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, B: 0.0001% to0.0050%, Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Cu: 0.001% to 2.0%, Ni:0.001% to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Zr: 0.0001% to0.2%, As: 0.0001% to 0.50%, Mg: 0.0001% to 0.010%, Ca: 0.0001% to0.010%, and REM: 0.0001% to 0.1% and balance composed of iron andinevitable impurities; second hot rolling in which a total of rollingreduction ratios is 50% or more is carried out in a temperature range ofT1+30° C. to T1+200° C.; third hot rolling in which a total of rollingreduction ratios is less than 30% is carried out in a temperature rangeof T1° C. to lower than T1+30° C.; and hot rolling ends at an Ar3transformation temperature or higher, where, T1 is a temperaturedetermined by steel sheet components, and expressed by the followingformula 1,T1(°C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (Formula 1)2. The method of manufacturing a hot-rolled steel sheet according toclaim 1, wherein, in the second hot rolling in the temperature range ofT1+30° C. to T1+200° C., the ingot or slab is rolled at least once at arolling reduction ratio of 30% or more in a pass.
 3. The method ofmanufacturing a hot-rolled steel sheet according to claim 1, wherein, inthe first hot rolling in a temperature range of 1000° C. to 1200° C.,the ingot or slab is rolled at least twice at a rolling reduction ratioof 20% or more, and the austenite grain diameter is set to 100 μM orless.
 4. The method of manufacturing a hot-rolled steel sheet accordingto claim 1, wherein, in a case in which the pass in which the rollingreduction ratio is 30% or more in the temperature range of T1+30° C. toT1+200° C. is defined as a large reduction pass, a waiting time t fromcompletion of a final pass of the large reduction pass to initiation ofcooling employs a configuration that satisfies the following formula 2,t1≦t≦t1×2.5  (Formula 2) where t1 is expressed by the following formula3;t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 3) where Tfrepresents a temperature after the final pass, and P1 represents arolling reduction ratio in the final pass.
 5. The method ofmanufacturing a hot-rolled steel sheet according to claim 4, wherein atemperature of the steel sheet increases by 18° C. or less between therespective passes of the second hot rolling in the temperature range ofT1+30° C. to T1+200° C.
 6. A method of manufacturing a cold-rolled steelsheet, the method comprising, pickling, after the end of the hot rollingthe hot-rolled steel sheet obtained through the method of manufacturingthe hot-rolled steel sheet according to claim 1 at the Ar3transformation temperature or higher; cold-rolling at 20% to 90%;annealing at a temperature range of 720° C. to 900° C. for a holdingtime of 1 second to 300 seconds; acceleration-cooling at a cooling rateof 10° C./s to 200° C./s from 650° C. to 500° C.; and holding at atemperature of 200° C. to 500° C.
 7. The method of manufacturing acold-rolled steel sheet according to claim 6, wherein, in the second hotrolling in the temperature range of T1+30° C. to T1+200° C., rolling ata rolling reduction ratio of 30% or more in a pass is carried out atleast once.
 8. The method of manufacturing a cold-rolled steel sheetaccording to claim 6, wherein, in the first hot rolling in thetemperature range of 1000° C. to 1200° C., rolling at a rollingreduction ratio of 20% or more is carried out at least twice, and theaustenite grain diameter is set to 100 μm or less.
 9. The method ofmanufacturing a cold-rolled steel sheet according to claim 6, wherein,in a case in which the pass in which the rolling reduction ratio is 30%or more in the temperature range of T1+30° C. to T1+200° C. is definedas a large reduction pass, a waiting time t from completion of a finalpass of the large reduction pass to initiation of cooling employs aconfiguration that satisfies the following formula 4,t1≦t≦t1×2.5  (Formula 4) where t1 is expressed by the following formula5;t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 5) where Tfrepresents a temperature after the final pass, and P1 represents arolling reduction ratio in the final pass.
 10. The method ofmanufacturing a cold-rolled steel sheet according to claim 9, wherein atemperature of the steel sheet increases by 18° C. or less between therespective passes of the second hot rolling in the temperature range ofT1+30° C. to T1+200° C.
 11. A method of manufacturing a galvanized steelsheet, the method comprising, a winding in a temperature range of 680°C. to room temperature, after the end of the hot rolling the hot-rolledsteel sheet obtained through the method of manufacturing the hot-rolledsteel sheet according to claim 1 at the Ar3 transformation temperatureor higher; pickling; cold-rolling at 20% to 90%; heating to atemperature range of 650° C. to 900° C.; annealing for a holding time of1 second to 300 seconds; cooling at a cooling rate of 0.1° C./s to 100°C./s from 720° C. to 580° C.; and galvanizing treating.
 12. The methodof manufacturing a galvanized steel sheet according to claim 11,wherein, in the second hot rolling in the temperature range of T1+30° C.to T1+200° C., rolling at a rolling reduction ratio of 30% or more in apass is carried out at least once.
 13. The method of manufacturing agalvanized steel sheet according to claim 11, wherein, in the first hotrolling in the temperature range of 1000° C. to 1200° C., rolling at arolling reduction ratio of 20% or more is carried out at least twice,and the austenite grain diameter is set to 100 μm or less.
 14. Themethod of manufacturing a galvanized steel sheet according to claim 11,wherein, in a case in which the pass in which the rolling reductionratio is 30% or more in the temperature range of T1+30° C. to T1+200° C.is defined as a large reduction pass, a waiting time t from completionof a final pass of the large reduction pass to initiation of coolingemploys a configuration that satisfies the following formula 6,t1≦t≦t1×2.5  (Formula 6) where t1 is expressed by the following formula7;t1=0.001×((Tf−T1)×P1)²−0.109×((Tf−T1)×P1)+3.1   (Formula 7) where Tfrepresents a temperature after the final pass, and P1 represents arolling reduction ratio in the final pass.
 15. The method ofmanufacturing a galvanized steel sheet according to claim 14, wherein atemperature of the steel sheet increases by 18° C. or less between therespective passes of the second hot rolling in the temperature range ofT1+30° C. to T1+200° C.